BREAKING CHANGE(server): disable legacy backends include MySQL/PG/c*(.etc)

[Pengzna]

as title.

sub task of #2724

in 1.7.0, we tend to disable cassandra, mysql, postgresql, cockroachdb or palo as backend.
only hstore, rocksdb, hbase, memory are supported for backend.

if user want to use cassandra, mysql, postgresql, cockroachdb or palo as backend, please find a version before 1.7.0 of apache hugegraph for your application.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.38%. Comparing base (c99cfcd) to head (265f2aa).
Report is 1 commits behind head on master.

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HEAD has 6 uploads less than BASE

Flag	BASE (c99cfcd)	HEAD (265f2aa)
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Additional details and impacted files

@@              Coverage Diff              @@
##             master    #2746       +/-   ##
=============================================
- Coverage     46.95%   34.38%   -12.57%     
+ Complexity      821      375      -446     
=============================================
  Files           745      745               
  Lines         60076    60068        -8     
  Branches       7672     7672               
=============================================
- Hits          28210    20656     -7554     
- Misses        29043    37128     +8085     
+ Partials       2823     2284      -539     

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[Copilot]

Pull Request Overview

This PR disables backends that are no longer supported in version 1.7.0 of HugeGraph by removing registrations, enforcing allowed backends in the core provider, updating documentation, and restricting backend testing in CI.

• Removed backend registration calls for Cassandra, ScyllaDB, MySQL, and Palo in the example utility.
• Enforced allowed backend values in the provider factory and updated the instantiation method.
• Updated both server and global README files and the CI workflow to reflect the supported backends.

Reviewed Changes

Copilot reviewed 7 out of 22 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
hugegraph-server/hugegraph-example/src/main/java/org/apache/hugegraph/example/ExampleUtil.java	Removed calls to register unsupported backends.
hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/backend/store/BackendProviderFactory.java	Added an ALLOWED_BACKENDS check, made providers final, and updated instance creation.
hugegraph-server/README.md	Updated supported backend list to display only allowed backends.
README.md	Revised backend support details in the plug-in backend driver section.
.github/workflows/server-ci.yml	Limited CI testing matrix to supported backends.

Files not reviewed (15)

• .github/outdated/.travis.yml: Language not supported
• hugegraph-cluster-test/hugegraph-clustertest-dist/src/assembly/static/conf/hugegraph.properties.template: Language not supported
• hugegraph-server/hugegraph-dist/docker/scripts/detect-storage.groovy: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/build.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/config.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/deploy.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/jenkins.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/publish.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/jenkins/test.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/static/conf/graphs/hstore.properties.template: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/static/conf/graphs/hugegraph.properties: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/travis/install-backend.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/assembly/travis/start-server.sh: Language not supported
• hugegraph-server/hugegraph-dist/src/main/resources/backend.properties: Language not supported
• hugegraph-server/hugegraph-example/src/main/resources/hugegraph.properties: Language not supported

Comments suppressed due to low confidence (1)

hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/backend/store/BackendProviderFactory.java:70

• [nitpick] The 'backend' variable is already lowercased upon retrieval; consider removing the redundant toLowerCase() call to improve clarity.

E.checkState(ALLOWED_BACKENDS.contains(backend.toLowerCase()),

[imbajin]

Remember to update the project version to 1.7.0 later (maybe in next PR?)