[8.8] Create delete-serverless-dir-in-backports.yml (backport #5300)

[mergify]

Resolves https://github.com/elastic/security-docs-internal/issues/18. Adds a GitHub action to keep serverless files from getting backported into versioned branches (for example, 7.17 or 8.15).

Why do this?

We don't want to add version-less serverless source files into versioned branches (7.x and 8.x), which could happen with backports of PRs containing changes to both serverless and classic/stateful. The only place where serverless source files should exist is in main.

How it works

1. The action is triggered by activity on any pull request that targets a branch matching the pattern 7.* or 8.* . The action doesn’t run for PRs targeting any other branches, such as main.
2. It checks for the existence of the docs/serverless subdirectory within the PR’s working branch.
3. If the directory doesn’t exist, nothing else happens.
4. If the directory exists, the action deletes it and its entire contents from the PR’s working branch in a single commit, which you can then confirm in the PR’s history.

Previews and testing

This was kinda difficult to test without first merging the workflow YAML and then testing it "live." But I did manage to test and validate that this works as expected. Expand the section below for more details.

The biggest testing takeaway was that we'll need to backport the workflow YAML file to all the previous branches where we want it to run. Just having the action in main isn't enough, because backport PRs don't target main. That makes total sense, but I didn't realize it until halfway through the tests.

Expand for gory details

• First I tested in a very simple private repo, but I didn't feel super confident about that because it was too simple to be realistic. It also wasn't in a good shareable state, so others couldn't verify my work or refer back to it if needed.

• Next I tested in my personal fork of security-docs. I created a few versioned 8.x and 7.x branches, added the YAML, and then opened some PRs to confirm that the action triggered and ran as expected (or didn't run when not expected to do so). Everything looks good!

  • Test a PR targeting the 8.14 branch joepeeples/security-docs#8
  • Test a PR targeting the main branch joepeeples/security-docs#9
  • Test a PR targeting the 8.14 branch, classic changes only joepeeples/security-docs#10

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This is an automatic backport of pull request #5300 done by [Mergify](https://mergify.com).

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