Switch to new note syntax

[ewjoachim]

Syntax for admonitions, including Note has changed: https://github.com/orgs/community/discussions/16925#discussioncomment-6506860

A new syntax, [!NOTE], has been added, which will gradually replace the old one. However, the old syntax will continue to work for some time.

We're using the old syntax here:

python-coverage-comment-action/coverage_comment/template_files/comment.md.j2

Line 18 in 16ccd40

> **Note**

Let's replace it.

(This may be made redundant by #211 but I'm just opening the issue to make sur we don't forget it if need be)