[Boost] Add "ignore" to the list of Critical CSS suggestions

[thingalon]

Ref: p1707134680670189-slack-C016BBAFHHS

We show "Advanced Recommendations" in the Boost UI when generating Critical CSS fails for some pages (but works for others). One recommendation we show is for pages that get a 301 error:

The instructions tell users a bunch of things to try, but they don't tell users that sometimes it's ok to ignore this error.

This can occur when:

• The user has installed a plugin that sets up redirect URLs, or
• The user has uninstalled a plugin that has left some stuff behind in WordPress.

Generally speaking, if the URL listed in the advanced recommendation isn't a useful part of their site, they should disregard the message.

We should add a step to the instructions, something like:

if this page is not a part of your site, then this error is safe to disregard.

[github-actions[bot]]

This issue has been marked as stale. This happened because:

• It has been inactive for the past 6 months.
• It hasn’t been labeled `[Pri] BLOCKER`, `[Pri] High`, `[Type] Feature Request`, `[Type] Enhancement`, `[Type] Janitorial`, `Good For Community`, `[Type] Good First Bug`, etc.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.