build: rename meson_options.txt to meson.options

...because:

- this is the preferred name since Meson 1.1
- wlroots did it

meson.build

@@ -3,7 +3,7 @@ project(
   'c',
   version: '0.8.2',
   license: 'GPL-2.0-only',
-  meson_version: '>=0.59.0',
+  meson_version: '>=1.1',
   default_options: [
     'c_std=c11',
     'warning_level=2',