kopia: update to 0.21.1.

[fapdash]

Testing the changes

• I tested the changes in this PR: YES

cc @daniel-eys since you've updated kopia in the past.

[fapdash]

The fix the linting error I've updated the commit to include a proper e-mail for committer and author.

Three platforms failed the build bc of failing checks:

• x86_64, x86_64, glibc, linux/amd64
• i686, i686, glibc, linux/386
• x86_64-musl, x86_64-musl, musl, linux/amd64

The other platforms succeeded because they skipped running the checks.

There seem to be two problems showing up:

1. ssh-keygen is missing from $PATH.
2. Tests that check for a specific output string expect the version to be Kopia v0-unofficial, but it's actually Kopia 0.21.1.

I've added two commits that should address these issues.