Unify email length checks

[rmccue]

Right now, the comment form checks for 6 characters minimum for email addresses. In #38506, we updated the REST API to do the same.

We should update is_email() to include this check, or change the other checks to match is_email(). It makes no real sense to have the check in two places.

[lukecavanagh]

@rmccue

Do you just mean in?

​https://github.com/WordPress/WordPress/blob/cf786e592c41487f6acbf47c2d178d9ac2e08b73/wp-includes/formatting.php#L2863-L2941

[rmccue]

@lukecavanagh Yep, specifically ​this line checking for length > 3 should be changed to > 6.

(Pro-tip: When on GitHub, hit "y" to get a permalink to lines linked to a specific commit, otherwise they can potentially change with future commits.)

[lukecavanagh]

@rmccue 

Good to know.

[lukecavanagh]

Basic patch.

[rachelbaker]

Adds tests and removes extra handling in rest-api

[rachelbaker]

In 38708.2.diff​ I added tests and removed the extra string length check and comments from r39158.
Can probably also remove the string length check in wp_handle_comment_submission()

[desrosj]

38708.patch​ removes the length check in wp_handle_comment_submission(). I also noticed a length check in sanitize_email(). Updated that one to 6 for consistency.

[sudar]

38708.2.patch just removes an unnecessary whitespace change from 38708.patch

[rachelbaker]

In 40667:

Formatting: Increase minimum characters allowed in is_email() to 6.

Brings the minimum characters expected for a valid email address to six, which matches the expectations in wp_handle_comment_submission() and REST API email arguments.

Props rmccue, lukecavanagh, rachelbaker, desrosj, sudar.
Fixes #38708.