mastodon: Update mastodon to v3.3.0

After some further investigation, it turns out, not the SAML
authentication itself is broken in Mastodon 3.3.0, just the automated
login using `OAUTH_REDIRECT_AT_SIGN_IN` results in a broken state.

The details of why, are still to be explored. However, it was possible
to prove, that the problem goes away, when setting this option to
`false`.

For further details see the bug report:
https://github.com/tootsuite/mastodon/issues/15959

This reverts commit 8d7a225e.