Date and time patterns should follow `DateTimeFormatter`

[ppkarwasz]

In release 2.x date time patterns follow SimpleDateFormat with additional extensions provided by FastDateFormat and support for the n specifier for nanoseconds.

The exception is JSON Template Layout, which uses the format specifiers from DateTimeFormatter.

In release 3.x we should switch all format specifiers to those documented DateTimeFormatter.

The major differences are:

• u means day-of-week in SDF and era in DTF,
• SDF lacks L meaning month as text,
• SDF lacks q and Q as quarter,
• in DTF Z, ZZ and ZZZ all mean the same format for the time zone (e.g. -0800, +0000). In SDF (Commons Land version) ZZ uses the ISO 8601 format with a colon (e.g. -08:00, Z) and ZZZ adds a colon to the original Z format (e.g. -08:00, +00:00).

[vy]

Even though this will be a (slightly?) breaking change, can't we rather implement this (breaking change) in 2.x and introduce a property to restore the old behaviour? As long as we clearly indicate this in release notes, this feels legit to me. @rgoers, WDYT?

[garydgregory]

SDT -> SDF?

[ppkarwasz]

Fixed by #3150.