feat: deprecate repo

[TheOutdoorProgrammer]

Summary

Adds deprecation notice to the repository, informing users that the sops plugin functionality is now natively available in Spacelift and this standalone module is no longer maintained.

Motivation

The sops plugin functionality has been integrated directly into Spacelift's native plugins feature. Users can now install and configure sops through the Spacelift UI or API, eliminating the need for this separate Terraform module. This deprecation notice ensures existing users understand the migration path and can transition to the native implementation.

Changes

• Added prominent deprecation warning banner at the top of README.md
• Redirected users to Spacelift's official documentation for current sops plugin usage
• Preserved original documentation under "Legacy Documentation" section for historical reference

Migration Path

Users of this module should:

1. Remove this Terraform module from their Spacelift configuration
2. Configure the sops plugin through the Spacelift UI or API instead
3. Refer to the Spacelift documentation for current implementation details