Redactions should preserve the `notifications`/`invite` key on power levels

Backstory: Notification power levels were added after redactions and hashes were introduced. This results in a protocol version bump being needed to change the redaction rules otherwise hashes would be wrong for servers, making the event fairly conflicted.

Start by reviewing the Matrix specification's redaction rules and the protocol version history; no file or test is named in the issue. Confirm the expected treatment of the `notifications`/`invite` key and the versioning implications, with completion requiring correct redaction behavior without invalidating event hashes.

Written by the indexing model from the issue text.