[ci] Pin gcovr < 8.4

There are some changes in gcovr version 8.4 that currently result in
incorrect missed lines. Pin to a lower version for the time being.

Closes #20047

Author: hahnjo

.github/workflows/test-coverage.yml

@@ -113,7 +113,7 @@ jobs:
         run: |
           dnf -y update
           dnf -y install lcov
-          pip3 install gcovr
+          pip3 install "gcovr<8.4"
 
       # This checks out the merge commit if this is a PR
       - name: Checkout