ci: setup release-please (#317)

##### SUMMARY

The current release process for this project requires multiple manual
local steps to prepare and then tag the release. This introduces the
standard release-please workflow we have in most of our integrations.

Release-please will open a new PR whenever there is a releasable change
and it updates the CHANGELOG & other files automatically in this PR.
Once the PR is merged a new tag is created.

This repository has some additional complexity as it uses
`antsibull-changelog` to maintain the user-facing changelog. This is
implemented ina second job in the `release-please` workflow which
executes when the release-please PR is updated and then runs
`antsibull-changelog release` against the version. The resulting changes
are pushed as a second commit to the same PR.

.pre-commit-config.yaml

@@ -98,11 +98,3 @@ repos:
         entry: scripts/integration-test-files.sh
         pass_filenames: false
         files: ^(scripts/integration-test-files.sh$|tests/integration)
-
-      - id: check-version-variables
-        name: check version variables
-        description: Ensure the version variables are in sync
-        language: system
-        entry: scripts/version-sync.sh
-        pass_filenames: false
-        files: ^(scripts/version-sync.sh$|galaxy.yml|plugins/module_utils/version.py)