Merge branch 'feature/readme-update-latest-release' into 'master'

[readme] Update latest release pipeline to 3.9

See merge request !3871

README.md

@@ -108,7 +108,7 @@ our publications:
 
 # Automated Testing / Test suite
 
-* Latest release (3.8): [![release build badge](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/badges/releases/3.8/pipeline.svg)](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/pipelines?page=1&scope=all&ref=releases/3.8)
+* Latest release (3.9): [![release build badge](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/badges/releases/3.9/pipeline.svg)](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/pipelines?page=1&scope=all&ref=releases/3.9)
 * Master branch (development / unstable): [![master build badge](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/badges/master/pipeline.svg)](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/pipelines?page=1&scope=all&ref=master)