As reported by Piotr in #1416 <https://github.com/apache/logging-log4j2/issues/1416> (and causing the JTL issue in #1420 <https://github.com/apache/logging-log4j2/issues/1420>), certain formatting identifiers are interpreted differently between `FixedDateFormat`, `FastDateFormat`, and Java's `DateTimeFormatter`. I propose the following *breaking change* for `2.x`: follow DTF syntax everywhere and introduce a property to restore the old behaviour. Objections?
(Please see #1416 <https://github.com/apache/logging-log4j2/issues/1416> for the list of differences between formatters.)
