[BUGFIX] Change default branch for git-review

When switching from `master` to `main` as default branch, it was
forgotten to change the `defaultbranch` value in `.gitreview` as
well.

Resolves: #97843
Releases: main
Change-Id: I84d011f86d0f165eeb9eccce757bb7fc32f54924
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75014


Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>