slab-release-push-action needs update to reflect updates to Slab API

[elkskim]

What's the issue?

• The Slab API has recently been updated to properly implement mutations for creating/updating posts with content in a meaningful way.

What's the fix?

• Seeing as the slab-release-push-action uses the syncPost mutation to create and update posts, a large percentage of the code will have to be refactored to use createPost and updatePostContent instead.

Are we in a hurry?

• Seemingly not. There's no reason to believe that the action is not functional as is, but refactoring it to use the intended mutations in the API should make the script somewhat more efficient and readable.