This commit adds a job to the `[Daily] Unit Tests with nightly Puppet
gem` workflow that notifies us the conclusion of it via Slack.

…fication-job

(MODULES-10815) Add Slack notification job

Due to unknown environment changes in GitHub Actions runners, we
encountered the following error:
`fatal: cannot create directory: Filename too long`

This commit allows git to use long paths in our GitHub Actions
workflows.

…paths-in-gh-actions

(maint) Allow git to use long paths in GitHub Actions

Add beaker-puppet to unify core module gemfiles

Create a separate group in the Gemfile for pdk and puppet-blacksmith
which are only used for releasing. In the workflow, avoid installing the
release group.

Pin puppet-module-posix-system and puppet-module-win-system to an older
version since the newer ones do not bundle some gems that we use in
acceptance (i.e. beaker-module_install_helper), causing tests to fail.

Update the workflow that tests with released Puppet gems to also test
with Puppet 7.

Remove testing with Puppet 5 from the workflows since it reached EOL.

Bump Ruby version in the static code analysis workflow to 2.6, as 2.5
will be EOL soon.

(MODULES-10945) Core module spring cleaning 2021

To avoid having to update this everytime we release a new agent
platform, it should be enough to specify the supported OS, without
specific versions. It is assumed that for each OS in metadata.json, the
versions supported are the same as what the agent itself supports.

sshkeys-core metadata

…cky-alma-modules

(MODULES-11167)(MODULES-11191) Add RockyLinux and AlmaLinux to sshkeys-core metadata

Includes REFERENCE.md ToC fixes for MODULES-8183.

(MODULES-11197) Update to pdk-templates 2.2.0

Release prep v2.3.0