GTK+: Turn assumptions from notes into warnings

Use a single warning to state the knowledge and minimum version
assumptions regarding GTK+ and libhandy rather than two consecutive
notes, making these assumptions more noticeable.
1 job from libhandy-0-0-4 in 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #3037
librem5
job

00:00:43