cleanup(*): use priority 10 for low priority

[fpvandoorn]

Now we can locally use priorities lower than this low-priority.

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list

[jcommelin]

Should this PR also open_locale classical wherever it makes sense? Or maybe that's better done in a separate PR?

[fpvandoorn]

That was done already in almost all of the cases in #1319.

Most places which still have local attribute manually is because they don't import tactic/localized, or they change the priority of another declaration at the same time.

[semorrison]

Nice and straightforward!