Remove webhook validation code

[ciarams87]

Proposed changes

Problem: The webhook validation code has been removed from the upstream Gateway API repo, causing import errors when we try and run the conformance tests against the main branch of the Gateway API repo.

Solution: Remove the web hook validation code from our repo, and remove the web hook assumption panics. Also add additional validation for case not covered by CEL validation (see kubernetes-sigs/gateway-api#2277)

Testing: Added additional unit tests where appropriate, ran the conformance test suite against the GW API main branch

Closes #1512

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.

NONE

[sjberman]

One validation is not supported by CEL and needs to be implemented manually: kubernetes-sigs/gateway-api#2277

[ADubhlaoich]

LGTM on the docs side of things.

[kate-osborn]

LGTM