Minor improvements to "rebase needed" workflow

[bsmth]

I have two suggestions to improve the rebase needed workflow:

Rebase wording:

Regarding the wording in https://github.com/mdn/workflows/blob/main/.github/workflows/pr-rebase-needed.yml

We should maybe rephrase that rebasing is needed. Per the CONTRIBUTING.md we say:

Don't use git rebase of main over your branch. Your changes are replayed on top of the current main branch at that point in time. This might confuse reviewers as notifications on GitHub lead to nowhere.

Should we say that the branch needs to be "updated from main" or the author must "resolve merge conflicts" instead as we discourage rebasing?

Comments:

We might want to reduce the number of new comments this is adding each time a rebase is needed. Alternatively, edit existing comments with a strikethrough when it's been resolved. The PR companion workflow is nice because there is one comment which is updated with the last-known state. See mdn/interactive-examples#2288 for example.

[bsmth]

Note that "sticky" pull request comment edits a comment in-place instead of adding new ones to a PR: https://github.com/marocchino/sticky-pull-request-comment

[bsmth]

I'm going to close this as part 1 (rebasing wording) is done. Tracking the rest in #120