Remove OAuth option to authenticate with password (again)

[bocops]

According to Trunk & Tidbits, October 2024, support for authenticating to the API using username and password is being removed with Mastodon v.4.3.1.

This presumably affects our OAuth method getUserAccessTokenWithPasswordGrant(), which should probably be marked as deprecated now and removed as soon as we no longer support Mastodon versions < 4.3.1.

In addition to that, there's a sample and the whole integration test package to be rewritten.