Ignore a few more false positives

What does this MR do and why?

Removes a few more false positives in the drive for https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/6610.

Commits

Ignore textarea placeholder finding

This is a unique case which doesn't need to use GlFormTextarea.

Closes #132 (closed).

Ignore loading skeleton false positive

Closes #131 (closed).

Ignore false positive submit button

Closes #133 (closed).

Review/run this locally

1. Copy the pages in mr job URL (must have succeeded)
2. Run bin/review-mr.sh <job url>
3. If there are rules changes, check the compare findings job log reports the expected changes
4. If there are group changes, check the compare groups job log reports the expected changes