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

Bumps [isomorphic-git](https://github.com/isomorphic-git/isomorphic-git) to 1.21.0 and updates ancestor dependency [@antora/site-generator-default](https://gitlab.com/antora/antora). These dependencies need to be updated together.


Updates `isomorphic-git` from 0.78.5 to 1.21.0
- [Release notes](https://github.com/isomorphic-git/isomorphic-git/releases)
- [Changelog](https://github.com/isomorphic-git/isomorphic-git/blob/main/docs/in-the-news.md)
- [Commits](https://github.com/isomorphic-git/isomorphic-git/compare/v0.78.5...v1.21.0)

Updates `@antora/site-generator-default` from 2.3.3 to 3.1.3
- [Release notes](https://gitlab.com/antora/antora/tags)
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v2.3.3...v3.1.3)

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

Signed-off-by: dependabot[bot] <support@github.com>
2 files changed