Make maintenance easier: ddev-drupal-contrib, drush, phpstan

Problem/Motivation

To make this module easier to maintain, I would like to make a few changes...

1. Update our GitLab CI config (let's take advantage of the community's latest changes)
2. Install ddev/ddev-drupal-contrib (Ddev is now Drupal's official development environment. Its ddev/ddev-drupal-contrib plugin makes developing/contributing to contrib modules a lot easier.)
3. Make it easier to use Drush to set up (i.e.: site-install) a development environment. (in ddev/ddev-drupal-contrib you install development dependencies by adding them to the require-dev section in composer.json.)
4. Set up PHPStan (PHPStan is a tool that does static analysis on a codebase to find bugs. PHPStan is run in our CI pipeline. PHPStan groups its lints into rule levels. Currently PHPStan doesn't report any issues because we don't have a configuration file for it yet.)

Proposed resolution

1. Update .gitlab-ci.yml. Note the latest version at time-of-writing is 1.15.0.
2. Run the ddev/ddev-drupal-contrib setup in this repo and commit the scaffolding to make setup easier. Note the latest version at time-of-writing is ddev/ddev-drupal-contrib 1.1.5.
3. Add "require-dev": {"drush/drush": "*"} to composer.json. This will allow any version of drush that supports whichever version of core a user happens to be testing the feedback module with.
4. Create phpstan.neon and phpstan-baseline.neon files and update them to the highest level we can without errors.

Remaining tasks

1. Merge request - merge request !3 created by @mparker17 in #2
2. Commit - merged by @mparker17 in #5
3. Release

User interface changes

None.

API changes

None.

Data model changes

None.