Bump ruby version for Chef cookbooks

### Goal

Update the pinned ruby version of our Chef cookbooks so that we can reliably run their `rspec` tests locally with `bundle exec rspec`.

### Changes to make

For each chef cookbook that has a pinned ruby version:
* Update the `.ruby-version` file to `2.6.3`.
* Update the `.gitlab-ci.yml` file to refer to `ruby` container image of the same major+minor version as in step 1.

For reference, the `gitlab-server` cookbook is definitely affected, and the following additional cookbooks may be affected:

```shell
cookbook-about-gitlab-com/.ruby-version:2.4.4
cookbook-license-gitlab-com/.ruby-version:2.4.4
cookbook-omnibus-gitlab/.ruby-version:2.5.3
gitlab-exporters/.ruby-version:2.5.3
gitlab-haproxy/.ruby-version:2.3.3
gitlab_lsyncd/.ruby-version:2.4.4
gitlab-nessus/.ruby-version:2.4.5
gitlab-omnibus-prerequisites/.ruby-version:2.3.6
gitlab-openssh/.ruby-version:2.3.6
gitlab-prometheus/.ruby-version:2.5.3
gitlab-server/.ruby-version:2.5.3
gitlab-uptycs/.ruby-version:2.4.5
```

### Background

Yesterday I discovered that due to a known bug in RubyGems, I could not run `bundler install` on a cookbook (which was a prerequisite to my goal at the time: running the cookbook's rspec tests locally with `bundle exec rspec`).  The bug and its work-arounds are described here:

https://bundler.io/blog/2019/05/14/solutions-for-cant-find-gem-bundler-with-executable-bundle.html

This bug requires your `bundler` ruby gem to *exactly* match the version of bundler that last wrote the Gemfile.lock (indicated by that file's `BUNDLED WITH` line).

If you are using a version of ruby with the defective RubyGems that does not allow newer versions of bundler, then any bundler command will fail.  12 of our chef cookbooks currently pin the ruby version using a `.ruby-version` file (which is used by rbenv to switch the active ruby version).  Updating the pinned ruby version in `.ruby-version` to be >= `2.6.3` would avoid this bug in RubyGems.  Our chef-repo itself is currently pinned to ruby 2.6.3, so I'd like to propose updating all 12 pinned cookbooks to use that same ruby version.

Note that our cookbooks' CI pipeline `test` stage does not seem to be affected by this bug, presumably because it always runs in a container starting with a pristine ruby environment.

Questions for the team:
1. Can anyone think of a reason not to do this?
2. Apart from running each cookbook's rspec tests and letting the CI pipeline run its integration tests, what other testing would be appropriate?  Some cookbooks lack rspec tests for some recipes.

#### Example of the problem

```shell
$ cat .ruby-version 
2.5.3

$ rbenv install
...

$ rbenv version
2.5.3 (set by /home/msmiley/src/git/gitlab/gitlab-cookbooks/gitlab-server/.ruby-version)

$ gem install bundler
Fetching: bundler-2.1.4.gem (100%)
Successfully installed bundler-2.1.4
Parsing documentation for bundler-2.1.4
Installing ri documentation for bundler-2.1.4
Done installing documentation for bundler after 3 seconds
1 gem installed

$ bundle install
Traceback (most recent call last):
	2: from /home/msmiley/.rbenv/versions/2.5.3/bin/bundle:23:in `<main>'
	1: from /data/src/git/public/rbenv/versions/2.5.3/lib/ruby/2.5.0/rubygems.rb:308:in `activate_bin_path'
/data/src/git/public/rbenv/versions/2.5.3/lib/ruby/2.5.0/rubygems.rb:289:in `find_spec_for_exe': can't find gem bundler (>= 0.a) with executable bundle (Gem::GemNotFoundException)

$ grep -A 1 'BUNDLED WITH' Gemfile.lock 
BUNDLED WITH
   1.17.2

$ gem install bundler -v "$(grep -A 1 "BUNDLED WITH" Gemfile.lock | tail -n 1)"
...

$ bundle install
Fetching gem metadata from https://rubygems.org/..........
...
```

Story points: 1.0