[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 <[email protected]>
Tested-by: core-ci <[email protected]>
Reviewed-by: Oliver Hader <[email protected]>

Author: eliashaeussler

.gitreview

@@ -2,4 +2,4 @@
 host=review.typo3.org
 port=29418
 project=Packages/TYPO3.CMS
-defaultbranch=master
+defaultbranch=main