FeatureDeprecated: add a notice summary of future deprecations
FeatureDeprecated: add a notice summary of future deprecations

Since these aren't warnings, per se, we don't note every single call
site that has one. And we raise mlog.notice in non-fatal mode to avoid
either:
- being too threatening
- making builds fail with --fatal-meson-warnings

Nevertheless, it is useful to give people a heads-up that there is an
upgrade opportunity, rather than waiting until they upgrade and then
causing projects to begin printing fatal warnings.

mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson