[doc] plugins: Add note about Heroku plugin sunset [ci skip]

[olleolleolle]

Description

This PR adds a note to the Puma docs that heroku plugin is no longer a current need.

I didn't change more of the text, since the example plugin "heroku" was good at not being a built-in plugin, so the examples were better off keeping it, until a new non-built-in plugin has been added.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added an entry to History.md if this PR fixes a bug or adds a feature. If it doesn't need an entry to HISTORY.md, I have added [changelog skip] or [ci skip] to the pull request title.
• I have added appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.