Bump rust-version to 1.73.0

There was a fix for the definition of the `stat` struct on Android in
1.73, and even though it's unlikely to affect many applications it still
seems worthwhile to draw a line under this and ensure that all
android-activity based applications will have that fix.

Rust 1.73 was released October 2023, which is still well over a year old
and very conservative.

This updates `generate-bindings.sh` to pass `--rust-target 1.73.0` so we
avoid generating bindings that require a more recent compiler.

(This doesn't actually regenerate the bindings but does ensure that
future updates will be constrained to generate code that is backwards
compatible with 1.73.)

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         # See top README for MSRV policy
-        rust-version: [1.69.0, stable]
+        rust-version: [1.73.0, stable]
     steps:
       - uses: actions/checkout@v4