Change: Deprecate legacy index format and `s3legacy` repository 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` repository 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 to the current layout using
`RESTIC_FEATURES=deprecate-s3-legacy-layout=false restic migrate s3_layout`.

It is possible to temporarily reenable support for the `s3legacy` layout 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
