Update `.github/workflows/` Files to Use Secrets Instead of Credentials

[aaronskiba]

Please complete the following fields as applicable:

What version of the DMPRoadmap code are you running? (e.g. v2.2.0)
4.1.1+portage-4.1.0-uat-5

Details:

• DMP Assistant favours using secrets over credentials. However, both .github/workflows/postgres.yml and .github/workflows/mysql2.yml include steps that entail the setup of credentials.

e.g. .github/workflows/postgres.yml

    # Stub out the Rails credentials file so that we can start the Rails app
    - name: 'Setup Credentials'
      run: |
        # generate a default credential file and key
        EDITOR='echo "$(cat config/credentials.yml.postgresql)" >' bundle exec rails credentials:edit

To better reflect the behaviour of DMP Assistant, and to give us more confidence in our tests, these workflows/ files should be updated to use secrets instead of credentials.