Deprecate the /new onboarding url & flow and replace with /start flow

[autumnfjeld]

Description

Once the /new vs /start experiment, 2bd76-pb/, is complete on March 18, and we've shown approximate parity in the performance of each according the experiment metrics, we can write up the "Add new site" button to the /start flow and deprecate the new flow.

Acceptance Criteria

• [x] All users will be directed to /start flow
• [x] We may need to keep the /new flow for users who link in from AnchorFM
• [x] Maybe: cleanup code that is no longer used (may not be applicable due to Anchor FM needing /new)
• [x] Connect with Paul Bonahora when removal of /new is "official" pbAok1-2zu-p2#comment-4896

Dev Tasks / list of PRs

• [ ] https://github.com/Automattic/wp-calypso/pull/62201

Related

"Add a new site" A/B testing /new vs /start #60166 Experiment: 2bd76-pb/ pbxNRc-1jD-p2

[autumnfjeld]

@roo2 Regarding AnchorFm & /new flow: paYKcK-1rX-p2 , left a comment there. cc @andres-blanco

[autumnfjeld]

Completed with https://github.com/Automattic/wp-calypso/pull/62201