chore(deps): update pre-commit hook executablebooks/mdformat to v0.7.21 (#695)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.pre-commit-config.yaml

@@ -48,7 +48,7 @@ repos:
   - id: pretty-format-toml
     args: [--autofix]
 - repo: https://github.com/executablebooks/mdformat
-  rev: 0.7.19
+  rev: 0.7.21
   hooks:
   - id: mdformat
     additional_dependencies: