Split Github workflows into individual files

To get a better overview of all the tasks that are being run as well
as for better maintainability, split the `test` Github workflow into
separate ones, one for executing the Rubocop checks and one per DB
we test against.

Since each DB tests against a grid of Ruby as well as DB versions, this
gives us a better view on which tests are still running.

Also, given that the 18.04 Ubuntu image has been deprecated by Github,
upgrade the Rubocop and Postgres jobs to run on Ubuntu 22.

For MySQL this is a bit more complicated since newer versions of Ubuntu
are incompatible with MySQL 5.7 but we still need to keep support for
5.7 as it still is a maintained MySQL version.

Therefore, use MacOS for testing against MySQL 5.7 and Ubuntu to test
MySQL 8.

But since MacOS does not allow to install the `pg` gem unless Postgres
is installed on the machine, remove the `pg` gem from the main Gemfile
and install it only in the Postgres test flow via a custom Gemfile. The
`mysql` gem is used for local testing and will therefore stay in the
main Gemfile for now.

.github/workflows/rspec-on-mysql.yml

@@ -7,12 +7,14 @@ on:
     branches: [ master ]
 
 jobs:
-  test-on-mysql:
+  # MySQL 5.7 is not supported on Ubuntu 22, therefore using MacOS for this
+  # version
+  test-on-mysql-5-7:
     runs-on: macos-13
     strategy:
       matrix:
-        ruby-version: ['2.7', '3.0', '3.1', '3.2']
-        mysql-version: ['5.7', '8.0']
+        ruby-version: ['2.7', '3.0', '3.1']
+        mysql-version: ['5.7']
 
     steps:
       - uses: actions/checkout@v2
@@ -32,3 +34,30 @@ jobs:
           DB_ADAPTER: mysql2
           DB_HOST: 127.0.0.1
           DB_USER: root
+
+
+  test-on-mysql-8:
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        ruby-version: ['2.7', '3.0', '3.1', '3.2']
+        mysql-version: ['8.0']
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ankane/setup-mysql@v1
+      with:
+        mysql-version: ${{ matrix.mysql-version }}
+    - name: Create the test database
+      run: mysqladmin create rails_cursor_pagination_testing
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby-version }}
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - name: Run tests
+      run: bundle exec rake spec
+      env:
+        DB_ADAPTER: mysql2
+        DB_HOST: 127.0.0.1
+        DB_USER: root