[bugfix]: aws_elasticache_serverless_cache user group removal fix

[sukumaar]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

This fix enables user_group_id removal from aws_elasticache_serverless_cache while modification/update.

Relations

Closes #42766

References

Output from Acceptance Testing

% make testacc TESTS=TestAccElastiCacheServerlessCache PKG=elasticache ACCTEST_PARALLELISM=2 ACCTEST_TIMEOUT=3880m
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
xargs: gofmt: No such file or directory
make: Running acceptance tests on branch: 🌿 f-elasticache-serverless-ug-removal-fix 🌿...
TF_ACC=1 go1.24.10 test ./internal/service/elasticache/... -v -count 1 -parallel 2 -run='TestAccElastiCacheServerlessCache'  -timeout 3880m -vet=off
2025/12/12 20:34:11 Creating Terraform AWS Provider (SDKv2-style)...
2025/12/12 20:34:11 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccElastiCacheServerlessCacheDataSource_Redis_basic
=== PAUSE TestAccElastiCacheServerlessCacheDataSource_Redis_basic
=== RUN   TestAccElastiCacheServerlessCacheDataSource_Valkey_basic
=== PAUSE TestAccElastiCacheServerlessCacheDataSource_Valkey_basic
=== RUN   TestAccElastiCacheServerlessCache_basicRedis
=== PAUSE TestAccElastiCacheServerlessCache_basicRedis
=== RUN   TestAccElastiCacheServerlessCache_basicValkey
=== PAUSE TestAccElastiCacheServerlessCache_basicValkey
=== RUN   TestAccElastiCacheServerlessCache_full
=== PAUSE TestAccElastiCacheServerlessCache_full
=== RUN   TestAccElastiCacheServerlessCache_fullRedis
=== PAUSE TestAccElastiCacheServerlessCache_fullRedis
=== RUN   TestAccElastiCacheServerlessCache_redisUpdateWithUserGroup
=== PAUSE TestAccElastiCacheServerlessCache_redisUpdateWithUserGroup
=== RUN   TestAccElastiCacheServerlessCache_redisUpdateWithUserGroupRemoval
=== PAUSE TestAccElastiCacheServerlessCache_redisUpdateWithUserGroupRemoval
=== RUN   TestAccElastiCacheServerlessCache_fullValkey
=== PAUSE TestAccElastiCacheServerlessCache_fullValkey
=== RUN   TestAccElastiCacheServerlessCache_description
=== PAUSE TestAccElastiCacheServerlessCache_description
=== RUN   TestAccElastiCacheServerlessCache_cacheUsageLimits
=== PAUSE TestAccElastiCacheServerlessCache_cacheUsageLimits
=== RUN   TestAccElastiCacheServerlessCache_engine
=== PAUSE TestAccElastiCacheServerlessCache_engine
=== RUN   TestAccElastiCacheServerlessCache_disappears
=== PAUSE TestAccElastiCacheServerlessCache_disappears
=== RUN   TestAccElastiCacheServerlessCache_tags
=== PAUSE TestAccElastiCacheServerlessCache_tags
=== CONT  TestAccElastiCacheServerlessCacheDataSource_Redis_basic
=== CONT  TestAccElastiCacheServerlessCache_redisUpdateWithUserGroupRemoval
--- PASS: TestAccElastiCacheServerlessCacheDataSource_Redis_basic (374.93s)
=== CONT  TestAccElastiCacheServerlessCache_full
--- PASS: TestAccElastiCacheServerlessCache_full (314.08s)
=== CONT  TestAccElastiCacheServerlessCache_redisUpdateWithUserGroup
--- PASS: TestAccElastiCacheServerlessCache_redisUpdateWithUserGroupRemoval (1143.29s)
=== CONT  TestAccElastiCacheServerlessCache_fullRedis
--- PASS: TestAccElastiCacheServerlessCache_redisUpdateWithUserGroup (529.18s)
=== CONT  TestAccElastiCacheServerlessCache_basicRedis
--- PASS: TestAccElastiCacheServerlessCache_fullRedis (324.88s)
=== CONT  TestAccElastiCacheServerlessCache_engine
--- PASS: TestAccElastiCacheServerlessCache_basicRedis (367.53s)
=== CONT  TestAccElastiCacheServerlessCache_tags
--- PASS: TestAccElastiCacheServerlessCache_tags (381.88s)
=== CONT  TestAccElastiCacheServerlessCache_disappears
--- PASS: TestAccElastiCacheServerlessCache_disappears (305.22s)
=== CONT  TestAccElastiCacheServerlessCacheDataSource_Valkey_basic
--- PASS: TestAccElastiCacheServerlessCacheDataSource_Valkey_basic (304.21s)
=== CONT  TestAccElastiCacheServerlessCache_description
--- PASS: TestAccElastiCacheServerlessCache_engine (1251.20s)
=== CONT  TestAccElastiCacheServerlessCache_cacheUsageLimits
--- PASS: TestAccElastiCacheServerlessCache_description (407.64s)
=== CONT  TestAccElastiCacheServerlessCache_basicValkey
--- PASS: TestAccElastiCacheServerlessCache_basicValkey (307.41s)
=== CONT  TestAccElastiCacheServerlessCache_fullValkey
--- PASS: TestAccElastiCacheServerlessCache_cacheUsageLimits (629.00s)
--- PASS: TestAccElastiCacheServerlessCache_fullValkey (385.12s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/elasticache	3677.354s
...

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