Truncate month names in release note headers

[colebow]

Description

I've just now realized that all of our months in headers for past release notes have been 3 characters, but since I've taken over writing them, I've been spelling out the whole month. This has been fine up until this month, but 'September' needs to be abbreviated to 'Sep' to avoid the sidebar wrapping onto a second line, and we should probably fix all the past notes for consistency's sake.

Non-technical explanation

Updating release notes.

Release notes

• This is not user-visible and no release notes are required.
• Release notes are required, please propose a release note for me.
• Release notes are required, with the following suggested text:

...kinda feels like maybe that release note selection isn't the best... might add "This is a docs change and no release notes are required."

[mosabua]

Good catch.

[findepi]

• This is not user-visible and no release notes are required.
• Release notes are required, please propose a release note for me.
• Release notes are required, with the following suggested text:

Thanks for using checkboxes here.
Where can I see the negative consequences of the fact that PR description contains checkboxes?