FLTK status before experimental drivers were removed

Removed platforms and drivers include:

- Android
- Pico
- PicoAndroid
- PicoSDL

These platforms and their drivers were experimental and undocumented.

They have been removed for maintenance reasons as discussed in
fltk.coredev: "FLTK 1.4.0 release schedule"
https://groups.google.com/g/fltkcoredev/c/PDbHTRpXVh0/m/JqboexZ_AwAJ

More information, for instance where to find further development
(if any), will be documented in file README.experimental.txt in
the FLTK 'master' branch.

Release 1.3.8

Release 1.3.7

Release 1.3.6

Release 1.3.6rc2

Release 1.3.6rc1

Release 1.3.4-2

Release 1.3.5

Release 1.3.5rc2

Release 1.3.5rc1