Use correct syntax for var names in headings

[clayton-cornell]

PR Description

This PR adds the variables back to headings (changed in #6242 ). The correct syntax for shortcodes in headings is documented here: https://grafana.com/docs/writers-toolkit/write/shortcodes/#param

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[rfratto]

Obvious attempt to artificially increase your commit count 😉

[grafanabot]

The backport to release-v0.39 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-6244-to-release-v0.39 origin/release-v0.39
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x d017470abd9364c10fb3ba70ae36e61c39378a60
# Push it to GitHub
git push --set-upstream origin backport-6244-to-release-v0.39
git switch main
# Remove the local backport branch
git branch -D backport-6244-to-release-v0.39

Then, create a pull request where the base branch is release-v0.39 and the compare/head branch is backport-6244-to-release-v0.39.