Check if we should make use of Deprecation header

[bocops]

According to Trunks & Tidbits March 2025, Mastodon now makes use of a Deprecation header as defined in RFC 9745 (documentation).

Check if we can/should make use of this header somehow.

[PattaFeuFeu]

To quote the relevant part (highlight by me):

The API now supports the Deprecation header from RFC9745. We recommend that application developers look for this header in responses, and display a warning when it is present in development mode, to detect usage of deprecated APIs. See the documentation for more information. — Github PR #34262 (by ClearlyClaire)

I think we should use it, yeah.