Change all instances of date() to gmdate()

[Rarst]

Use of date() in core depends on PHP timezone set to UTC (performed by the core boot) and not changed by third party code (which cannot be guaranteed).

All instances of date() in core should be replaced with gmdate(), which is functionally equivalent but is not affected by PHP time zone setting (it's always UTC which is the exact behavior we need).

One of the steps for eliminating dependency on UTC timezone setting, see #44491

[SergeyBiryukov]

Previously: #37440

[Rarst]

I spotted that couple of changes are in upstream-ish libs (PHPMailer, SimplePie) so we might want to skip those. Otherwise looks good to me.

[Rarst]

I have rebased patch on latest master, threw out some white space changes that sneaked in, and ommitted couple changes in semi-external pieces for now.

[SergeyBiryukov]

In 45424:

Date/Time: Replace all instances of date() with gmdate().

Use of date() in core depends on PHP timezone set to UTC and not changed by third party code (which cannot be guaranteed).

gmdate() is functionally equivalent, but is not affected by PHP timezone setting: it's always UTC, which is the exact behavior the core needs.

Props nielsdeblaauw, Rarst.
Fixes #46438. See #44491.

[SergeyBiryukov]

In 47080:

Tests: Replace date() with gmdate() in WP_Test_REST_Posts_Controller tests.

Follow-up to [45424], [46252].

See #46438, #44491, #39953.