Change: Deprecate legacy index format and s3legacy layout

Support for the legacy index format used by restic before version 0.2.0 has
been deprecated and will be removed in the next minor restic version. You can
use `restic repair index` to update the index to the current format.

It is possible to temporarily reenable support for the legacy index format by
setting the environment variable
`RESTIC_FEATURES=deprecate-legacy-index=false`. Note that this feature flag
will be removed in the next minor restic version.

Support for the s3legacy layout used for the S3 backend before restic 0.7.0
has been deprecated and will be removed in the next minor restic version. You
can migrate your S3 repository using `RESTIC_FEATURES=deprecate-s3-legacy-layout=false restic migrate s3_layout`.

It is possible to temporarily reenable support for the legacy s3layout by
setting the environment variable
`RESTIC_FEATURES=deprecate-s3-legacy-layout=false`. Note that this feature flag
will be removed in the next minor restic version.

https://github.com/restic/restic/issues/4602
https://github.com/restic/restic/pull/4724
https://github.com/restic/restic/pull/4743
