Cleanup and revise release process

Summary:
Parts of the release process are outdated or don't make sense.
Changes include:

• Revision of "Check configuration" section, as it does not check anything configuration related.
• Moved the "Verify tests passed" section to after the "Update documents / code..." since tests should always be run on the release candidate.
• Moved items in the wrong sections to places that make more sense.
• Removed references to disk size expectations, as these were duplicated, outdated, and are now handled by automation.
• Reworded notes on updating chainparams and seeds.
• Simplified pushing to Github since the commits are automated.
• Removed the note about updating the version number on bitcoinabc.org since /latest effectively does this for us.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7182