Simplify mysql_to_rfc3339() and document RFC3339 incompatibility

[Rarst]

Erasing time zone with regular expression is redundant, since date can be just formatted in respective format.

Documentation needs correction since output is absolutely not compatible with RFC3339, only the more lax ISO8601 upstream from it.

[SergeyBiryukov]

Introduced in [34846].

[SergeyBiryukov]

In 43383:

Docs: Correct description for mysql_to_rfc3339().

Despite historical function name, the output does not conform to RFC3339 format, which must contain timezone.

Props Rarst.
See #42542.

[SergeyBiryukov]

In 43384:

Date/Time: Simplify mysql_to_rfc3339().

Erasing timezone with a regular expression is redundant, the date could be just formatted in the respective format instead.

Props Rarst.
Fixes #42542.