Bump version to v6.7.0 in prep for release (#4444)

## Summary
- Bump `version.json` from `v6.7.0-rc4` to `v6.7.0` to cut the final
`v6.7` release for mainnet. There is no rc5: this goes straight from rc4
to the release. (v6.6.0 was tagged on the rc5 bump, #3783, with
`version.json` still reading `v6.6.0-rc5`; this keeps `version.json` in
step with the tag.)

Contents since rc4: #4442 and #4439, plus the changelog update (#4443).
Merge this after #4443's backport has landed on `release/v6.7`, so the
`v6.7.0` tag includes the updated changelog.

- Both are labeled `non-app-hash-breaking` (a FlatKV migration metrics
fix and `seidb` digest tooling), so moving from rc4 to `v6.7.0` should
not need a coordinated validator switch.
- Push `v6.7.0` by hand on this PR's merge commit once it has merged;
the tagging ruleset stops `uci-release-publish` from creating it.

## Test plan
- [x] `git diff --check`

Made with [Cursor](https://cursor.com)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: alexander-sei

version.json

@@ -1,3 +1,3 @@
 {
-  "version": "v6.7.0-rc4"
+  "version": "v6.7.0"
 }