Date and Release fields should be removed from prev_branch's whatsnew when creating new_branch's whatsnew

[zware]

Somewhere around

release-tools/run_release.py

Lines 865 to 870 in a8894a3

	prev_branch = f"{release_tag.major}.{release_tag.minor}"
	new_branch = f"{release_tag.major}.{int(release_tag.minor)+1}"
	whatsnew_file = f"Doc/whatsnew/{new_branch}"
	with cd(db["git_repo"]), open(whatsnew_file, "w") as f:
	f.write(WHATS_NEW_TEMPLATE.format(version=new_branch, prev_version=prev_branch))

we can automatically remove the :Release: and :Date: fields from the previous branch's whatsnew to avoid issues like python/cpython#100700 and python/cpython#73905.

[oliver-rew]

Any qualms with me taking this one?

[hugovk]

Please go ahead, thank you!