add v2 deprecation warning

[nickfyson]

This PR adds a deprecation warning when v2 of any CodeQL Action is found to be running on a GitHub instance that is capable of upgrading to v3.

The following three workflow runs demonstrate these changes...

• Run using v2 of the action
• Run with f72cffc cherry-picked, adding the warnings
• Run with a854253 also cherry-picked, demonstrating that now only a single warning is shown.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

Suggested change

	* Prompt the customer to upgrade to CodeQL Action v2, if appropriate.
	* Prompt the customer to upgrade to CodeQL Action v3, if appropriate.

[henrymercer]

We could optionally set an environment variable such that we only show this warning for the first Action you run, so it only appears once in the workflow logs (at least per job).

[nickfyson]

I'd wondered whether we should be including the action name in the warning message, so in general users would still get multiple warnings for a job but each would refer to a specific action. Makes things more explicit? 🤷‍♂️

[nickfyson]

(though, we're linking to the changelog where the individual actions are listed, so probably is better to just limit ourselves to one warning...)

[henrymercer]

We could change the phrasing to something like "Please update all occurrences of the CodeQL Action in your workflow files to v3"?

[nickfyson]

👍 I've pushed updates, and included links in the description that demonstrate that the limit to one warning is working.