If a commit message currently mentions the breaking change or
deprecation note keywords, the commit message parse logic
accidentally picks up the note. This could then accidentally
prevent the commit from being merged (e.g. if the commit targets
the patch branch but mentioned the `BREAKING CHANGE: ` marker).
This commit switches the commit message notes pattern to only
capture notes at the beginning of a line (also allowing accidental
whitespace). This matches with the format we describe in our
contribution guide, as well as with our commit message validation
logic that also assumes notes at the beginning of a line.
PR Close#42436
d3531a7d41