meson: specify minimal target meson version for rust subprojects

Silences several meson warnings like:
```
../subprojects/equivalent-1.0.1/meson.build:9: WARNING: Project does not target a minimum version but uses feature introduced in '1.3.0': rust_abi arg in static_library.
```

The target of 1.7.0 was chosen since that's the minimal required meson version of the rust components in mesa anyway.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38814>

subprojects/packagefiles/unicode-ident-1-rs/meson.build

@@ -3,6 +3,7 @@ project(
   'rust',
   version : '1.0.12',
   license : '(MIT or Apache-2.0) AND Unicode-DFS-2016)',
+  meson_version : '>= 1.7.0',
 )
 
 lib = static_library(