docs: Add permissions stanza to manifest-releaser github script

[TimothyJones]

Github tightened the permissions sometime last year, so you now need to tell it to give the action permissions to modify and create PRs.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #1925