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.
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[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 (777fb59) to head (7f39b07).
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       22250    22251    +1     
  Branches     3591     3592    +1     
=======================================
+ Hits        14561    14562    +1     
  Misses       5888     5888           
  Partials     1801     1801           

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

... and 6 files with indirect coverage changes

[cwperks]

Suggested change

	"GET is not supported for /cache endpoint and will be removed in the next major version."
	"GET is not supported for /cache endpoint and will be removed in the next major version. Use DELETE instead."