Upgrade markPullRequestsAsDeployed to Node 16

[roryabraham]

Problem

The markPullRequestsAsDeployed action is still using Node 12

Why this is important

GitHub has deprecated Node 12 actions, and they will soon stop working.

Solution

Upgrade the action to Node 16, like all our other actions.

[roryabraham]

Put a PR up for this

[roryabraham]

Reopening until after our next deploy happens so we can verify it continues to work as expected.

[roryabraham]

This is working