Rename `FixKind` to `FixAvailability` (#7658)
**Summary** `FixKind` feels to generic, i suggest renaming it to
something like `FixAvailibility`.
Commands used:
```bash
rg FixKind --files-with-matches | xargs sed -i 's/FixKind/FixAvailability/g'
rg fix_kind --files-with-matches | xargs sed -i 's/fix_kind/fix_availability/g'
rg FIX_KIND --files-with-matches | xargs sed -i 's/FIX_KIND/FIX_AVAILABILITY/g'
cargo fmt
```
`rg -i "fix.kind"` doesn't show any matches anymore.
