Add deprecation warning for GET/POST/PUT cache

[derek-ho]

Description

Adds deprecation warning that GET/POST/PUT methods for cache endpoint is removed in version 3.0.0.

Issues Resolved

None
Is this a backport? If so, please add backport PR # and/or commits #, and remove backport-failed label from the original PR.
No

Do these changes introduce new permission(s) to be displayed in the static dropdown on the front-end? If so, please open a draft PR in the security dashboards plugin and link the draft PR here
No

Testing

None

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[DarshitChanpura]

LGTM!

[DarshitChanpura]

If you are instead trying to Deprecate the routes itself, you can do something like this:

https://github.com/opensearch-project/security/blob/main/src/main/java/org/opensearch/security/dlic/rest/api/SecuritySSLCertsApiAction.java#L60

waiting for deprecatedroute to be added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.44%. Comparing base (7ebcc00) to head (19706d1).
Report is 2 commits behind head on 2.x.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #4776   +/-   ##
=======================================
  Coverage   65.44%   65.44%           
=======================================
  Files         319      319           
  Lines       22252    22252           
  Branches     3594     3594           
=======================================
  Hits        14563    14563           
+ Misses       5887     5883    -4     
- Partials     1802     1806    +4     

Files with missing lines	Coverage Δ
...ch/security/dlic/rest/api/FlushCacheApiAction.java	65.38% <ø> (ø)

... and 6 files with indirect coverage changes