deprecate TEMPLATEPATH and STYLESHEETPATH

[abhansnuk]

Trac ticket: https://core.trac.wordpress.org/ticket/18298

Owned by; flixos90

[github-actions]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.

[codente]

We've asked for a dev note here:
https://core.trac.wordpress.org/ticket/18298#comment:77

Comment 68 has a good summary it appears
https://core.trac.wordpress.org/ticket/18298#comment:68

[nalininsbs]

As certain fields are not available to the docs release group, an additional note providing an overview of performance tickets in the run up to the Field Guide deadline, can be found on this docs sheet: Google sheet to better understand status on the performance labelled tickets. Note performance team is wrangling those tickets for identification, first draft and performance first review.