Merge two similar "terms" strings in the REST API

[ramiy]

See the attached patch.

[ocean90]

Should we also change Term doesn't exist. to Term does not exist. for consistency, see #38913?

[ramiy]

Replying to ocean90:

Should we also change Term doesn't exist. to Term does not exist. for consistency, see #38913?

Done in the second patch.

[rachelbaker]

In 39648:

REST API: Merge similiar error message strings in the Terms Controller.

Replaces the "doesn't" contraction with "does not" to be consistent with similar strings in WP_Error messages when a specified term or parent term is missing in WP_REST_Terms_Controller.

Props ramiy, ocean90.
Fixes #39176.

[audrasjb]

In 52978:

Administration: Replace contracted verb forms for better consistency.

This changeset replaces contracted verb forms like doesn't, can't, or isn't with non-contracted forms like does not, cannot, or is not, for better consistency across the WordPress administration. It also updates some corresponding unit tests strings.

Props Presskopp, socalchristina, aandrewdixon, francina, SergeyBiryukov, JeffPaul, audrasjb, hellofromTonya.
Fixes #38913.
See #39176.