meson: rename meson_options.txt to meson.options

Which is supported since Meson 1.1:
https://mesonbuild.com/Release-notes-for-1-1-0.html#support-for-reading-options-from-mesonoptions

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8651>

subprojects/gst-docs/markdown/installing/building-from-source-using-meson.md

@@ -328,7 +328,7 @@ rust plugins in `gst-build` (`rs` option), you would do:
 meson -Drs=disabled <build-directory>
 ```
 
-You can also peek at the `meson_options.txt` files and `subproject/xyz/meson_options.txt`
+You can also peek at the `meson.options` files and `subproject/xyz/meson.options`
 files which is where the various project specific build options are listed.
 These do not include all the standard Meson options however.