(MAINT) Add vendor/ to .gitignore

Following a failed attempt to run a release_prep workflows, we
realised that the vendor/ path was missing in the .gitignore file.
This caused the release process to add thousands of files to the
commit, which was not the intended behaviour.

Author: LukasAud

.gitignore

@@ -8,6 +8,7 @@
 /spec/reports/
 /tmp/
 *.gem
+vendor/
 
 # rspec failure tracking
 .rspec_status