Speed up and simplify PR pipelines

[ciarams87]

Proposed changes

Problem: Our CI run on PRs run in excess of 30 jobs on every update - this is slow, resource intensive, and likely overkill for the majority of PRs. Additionally, it increases the likelihood of having flaky test failures - often happens for 1/8 or 1/16 of the functional or conformance tests respectively - requiring rerunning of the failed jobs.

Additionally, Plus UBI builds fail pretty consistently in PRs from forks.

Each pipeline run also takes a really long time to build the binary and images.

Solution:
On PR runs:

• Run a reduced set of jobs and run the full matrix on main and release branches only.
• Only build the binary and images for amd64 and build for both supported architectures on main and release branches only.

On all runs:

• Remove the cache buster as it no longer works (they haven't been updated since April) and they incorrectly invalidate the cache for each job, meaning the binary was rebuilding for the conformance and functional tests
• Run binary build and unit tests in parallel

Testing: This PR pipeline

Please focus on (optional): Is running a subset of the tests on PR runs an acceptable trade off? Note we will have to update our required checks if we go with this option

Closes #ISSUE

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (97ebb04) to head (7e775bd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4219      +/-   ##
==========================================
- Coverage   86.01%   85.98%   -0.03%     
==========================================
  Files         131      131              
  Lines       14111    14111              
  Branches       35       35              
==========================================
- Hits        12138    12134       -4     
- Misses       1771     1773       +2     
- Partials      202      204       +2     

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