doc: release notes: add note about FP Kconfig symbol name changes

We add a note in the Zephyr v2.3.0 release notes to highlight
a renaming in the Floating Point Services main Kconfig options,
which was done in the 2.3 release cycle.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>