* DEV: Update Bundler
Latest RubyGems 3.1.1 vendors bundler 2.1.0 *again*. And our base
image build system even updates it to 2.1.1.
After that it is unable to run a simple `bundle install` because of
version mismatch.
Updating bundler to the one that comes with our enforced Ruby version
solves this.
* DEV: Update bundler in CI too