pages build step "Build with Jekyll" emits warnings about bundler

[bocytko]

Task Headline

https://github.com/zalando/zalando.github.io/actions/runs/6625406926/job/17996328161

Warning: the running version of Bundler (2.1.4) is older than the version that created the lockfile (2.3.26). We suggest you to upgrade to the version that created the lockfile by running `gem install bundler:2.3.26`.
The following gems are missing
 * minitest (5.19.0)
 * activesupport (7.0.6)
 * execjs (2.8.1)
 * ffi (1.15.5)
 * faraday (2.7.10)
 * nokogiri (1.15.3)
 * parallel (1.23.0)
 * rainbow (3.1.1)
 * yell (2.2.2)
 * zeitwerk (2.6.[11](https://github.com/zalando/zalando.github.io/actions/runs/6625406926/job/17996328161#step:4:12))
 * html-proofer (4.4.3)
 * webrick (1.8.1)
Install missing gems with `bundle install`
Warning:  github-pages can't satisfy your Gemfile's dependencies.
To use retry middleware with Faraday v2.0+, install `faraday-retry` gem
        Pagination: Pagination is enabled, but I couldn't find an index.html page to use as the pagination template. Skipping pagination.

Seems that it's also related to the action itself https://github.com/actions/jekyll-build-pages/issues/104.

Expected delivery

Builds pass w/o warnings.