chore: update to support GK 3.15 & remove support for GK 3.12

[akashsinghal]

Description

What this PR does / why we need it

Updates project support matrix to include 3.15.0 of Gatekeeper
Removes support for Gatekeeper 3.12.0

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.73%. Comparing base (45c0f81) to head (483c061).
Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1318      +/-   ##
==========================================
+ Coverage   61.60%   64.73%   +3.13%     
==========================================
  Files          98       98              
  Lines        6248     5059    -1189     
==========================================
- Hits         3849     3275     -574     
+ Misses       2066     1444     -622     
- Partials      333      340       +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.