Change: Disable S3 anonymous authentication by default

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

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

It is possible to temporarily 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
