Added milestone manager workflow

[weihanson07]

Purpose

Closes #584 .
Added a workflow to create and assign milestones to PRs following merges.

New Changes

• Wrote a workflow which triggers on merges to main
• Included automatic milestone assignment to PRs to track which year and term they were finished in
• Milestones will now be automatically created for the current term if they do not yet exist

Testing

Explain tests that you ran to verify code functionality.

• I have unit-tested this PR. Otherwise, explain why it cannot be unit-tested.
• I have tested this PR on a board if the code will run on a board (Only required for firmware developers).
• I have tested this PR by running the ARO website (Only required if the code will impact the ARO website).
• I have tested this PR by running the MCC website (Only required if the code will impact the MCC website).
• I have included screenshots of the tests performed below.

Testing was done in a separate repository where pull requests from multiple branches were merged into main. For the first successful test, the 25F milestone was automatically created and assigned. Subsequent pull requests were also assigned the 25F milestone.

Outstanding Changes

If there are non-critical changes (i.e. additional features) that can be made to this feature in the future, indicate them here.

[github-actions]

Pull reviewers stats

Stats of the last 120 days for UWOrbital:

	User	Total reviews	Time to review	Total comments
	Adityya-K 🥇	59 ▀▀▀▀▀	1d 14h 37m	187 ▀▀▀▀
	camspec 🥈	36 ▀▀▀	1d 14h 20m	232 ▀▀▀▀
	kepler452b123 🥉	14 ▀	6d 5h 35m ▀▀	37 ▀
	proprogrammer504	7 ▀	6d 17h ▀▀	24
	sunray4	3	4d 21h 20m ▀	19
	Yarik-Popov	1	22m	23
	c4bae	1	13d 1h 35m ▀▀▀▀	2
	Syzygicality	1	2d 22h 56m ▀	1

^{⚡️ Pull request stats}

[camspec]

lgtm!