build: bump required meson version
install arg in configure_file was added in 0.50.0
I keep getting
WARNING: Project specifies a minimum meson_version '>= 0.49.0' but uses features which were added in newer versions:
* 0.50.0: {'install arg in configure_file'}
with those build options
Tests: false
Examples: false
Documentation: false
Introspection: true
Vapi: false
Glade Catalog: false
------
and Project version: 0.80.0