Bump isomorphic-git and @antora/site-generator-default

Removes [isomorphic-git](https://github.com/isomorphic-git/isomorphic-git). It's no longer used after updating ancestor dependency [@antora/site-generator-default](https://gitlab.com/antora/antora). These dependencies need to be updated together.


Removes `isomorphic-git`

Updates `@antora/site-generator-default` from 2.3.4 to 3.1.9
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v2.3.4...v3.1.9)

---
updated-dependencies:
- dependency-name: isomorphic-git
  dependency-type: indirect
- dependency-name: "@antora/site-generator-default"
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>