Deprecate support for libtiff < 4

[radarhere]

libtiff 4.0 was released on 21st December 2011, 12 years ago.

I'm hoping that's long enough to not require a deprecation period for removing support for libtiff < 4.

[hugovk]

And looks like the last v3 release was v3.9.7, released in 2012:

• https://www.asmail.be/msg0054931648.html
• https://gitlab.com/libtiff/libtiff/-/tags?page=2
• http://www.simplesystems.org/libtiff/releases/index.html

If we make an exception to "at least one year of deprecation", I suggest we at least deprecate in Pillow 10.4.0 (July) and remove in 11.0.0 (October). I'd rather play it safe, so there's some warning period, and we still remove it in a SemVer major bump.

[radarhere]

If we're playing it safe, then I'm happy enough to just keep the normal 1 year minimum deprecation.

I've updated the commit to deprecate in Pillow 12.

[hugovk]

Thanks, let's also add to the 10.4.0 release notes.