chore: Deprecate san-pinning flag

[Tomasz-Smelcerz-SAP]

Description
See: #2966
This PR cleans up the related code, documentation and marks the enable-domain-name-pinning flag as deprecated.

Changes proposed in this pull request:

• Cleanup unused code
• Cleanup the docs
• Mark the flag as deprecated

Related issue(s)
#2966

[github-actions]

❌ Manifests created with 'make dry-run-control-plane' changed! Please make sure to check if changes are needed in related repositories like management-plane-charts, runtime-watcher, etc.

[c-pius]

I am a bit surprised why parts of the san pinning functionality have been removed here already, but not entirely? #2961

do you know why we did that?

[Tomasz-Smelcerz-SAP]

I am a bit surprised why parts of the san pinning functionality have been removed here already, but not entirely? #2961

do you know why we did that?

I think it was simply overlooked/forgotten by the author. Or it was intentionally left there, because we have a dedicated cleanup issue: #2440. @medmes any comment on that?
I noticed these parts should be removed while reviewing the original PR. But then it became obvious that the flag is no longer necessary.
To avoid making the PR even bigger and adding additional concerns to it, I decided It was simpler and faster to approve the PR as it was, and collect these left-overs here.

[medmes]

The reason was to make the PRs smaller and iterative, and as the cleanup overlap with the ticket verifyFunc it was decided to not remove it at that stage.