github: notify Slack when CI fails on merge to main/release.

[jrasell]

Description

This change creates a reusable workflow for notifying Slack on CI failures. The message will include useful links and information about the failure, so product engineers can investigate and fix any problems.

The new workflow is used by selected workflows which trigger on merges to main or release/* branches. The notification is only sent on failure and when the event was a push (PR merge) meaning the number of notifications should be minimal.

The aim is to help identify and draw attention to failure across our release branches, in particular when automated processes happen.

Links

Jira: https://hashicorp.atlassian.net/browse/NET-11243

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[tgross]

LGTM