Deprecate s3legacy layout

[MichaelEischer]

What does this PR change? What problem does it solve?

Disable support for the s3legacy layout to allow us to finally remove the code in a future restic version.

The local and sftp backend were also able to read repositories using the s3legacy layout. There is currently no documented migration path for such repositories, however, it is completely unclear whether such repositories even exist. I did not implement an upgrade path in restic, as the layout upgrade can also be handled using a short bash script if necessary.

Was the change previously discussed in an issue or on the forum?

Fixes #4602

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[MichaelEischer]

LGTM.