Change: Disallow S3 anonymous authentication by default

When using the S3 backend with anonymous authentication, it continuously tried
to retrieve new authentication credentials, which caused bad performance.

Now, to use anonymous authentication, it is necessary to pass the option `-o
s3.unsafe-anonymous-auth=true` to restic.

It is temporarily possible to revert to the old behavior by setting the
environment variable `RESTIC_FEATURES=explicit-s3-anonymous-auth=false`. Note
that this feature flag will be removed in the next minor restic version.

https://github.com/restic/restic/issues/4707
https://github.com/restic/restic/pull/4908