Add permissions block to changeset workflows

[Abram Sanderson (Abe27342)]

Description

Updates the workflows related to changeset management to include a permissions block.

See github permissions doc here.

[Abram Sanderson (Abe27342)]

Going to use this PR to validate the flow still works. I'll add links here as I do so to the relevant action runs.

[github-actions]

This PR requires a changeset, but none is present. You should add one before you merge. See the documentation in the wiki for more information about changesets and how to create them.

[Abram Sanderson (Abe27342)]

Upon adding changeset required label, workflows succeeded in posting a comment when corresponding PR didn't have a changeset. See above comment. Relevant action runs:

https://github.com/microsoft/FluidFramework/actions/runs/7632044704/job/20791366400?pr=19335
https://github.com/microsoft/FluidFramework/actions/runs/7632057995

Note that the second action isn't actually running the version with restricted permissions; commit it references is from main. After reading the workflow_run doc, I think this can only be tested by checking this change into a fork and verifying it works there. I've done that on this PR in my fork, see corresponding changeset-reporter run here.