Fix various codebase rots (stale CI, new Rust lints, broken MSRV checks by transitive dependency upgrades) (#164)

* game_activity/ffi: Drop cfg for inexistant `target_arch = "armv7"`

[Rust 1.80 from July 25th 2024] points out that `armv7` is not a known,
valid value for the `target_arch` cfg variable.  This is confirmed by
the docs not listing it either:
https://doc.rust-lang.org/reference/conditional-compilation.html#target_arch

Hence drop this entirely, and rely purely on `target_arch = "arm"`.

[Rust 1.80 from July 25th 2024]: https://blog.rust-lang.org/2024/07/25/Rust-1.80.0.html

* Fix `unexpected-cfgs` by adding `api-level-30` feature and removing `test`

Some code copied from the NDK carried over the respective `feature`
`cfg` guards, without ever adding the feature to the `[features]` list
in `Cargo.toml`.  Now that Rust detects these mishaps, we can fix it
by removing `test` (bindings don't seem to be run-tested) and reexpose
`ConfigurationRef::screen_round()` which was behind a previously
unsettable `feature = "api-level-30"`.

Also remove `unsafe impl Send/Sync for ConfigurationRef` since the
upstream `ndk` already declares `Configuration` to be `Send` and `Sync`,
and `RwLock` and `Arc` carry that through.

* native_activity: Fix clippy lints around `NativeActivityGlue` not `SendSync` and unwritten `redraw_needed` field

* CI: Remove deprecated/unmaintained `actions-rs` toolchain setup

The `actions-rs` containers haven't been maintained and updated for
years and don't need to: GitHub's actions environment already comes
fully loaded with a complete `stable` Rust installation with the
standard tools (in this case `rustfmt`).  Remove the remaining toolchain
setup (which was already replaced with `hecrj/setup-rust-action`
elsewhere) to get rid of ancient Node 12 deprecation warnings.

* Bump dependency patch-versions to fix `-Zminimal-versions` and MSRV check

Use `-Zminimal-versions` in our MSRV check.  This not only ensures
our minimum version bounds are actually solid and tested (even if
they may be a bit conservative at times, i.e. we could allow older
versions except for the crates that are bumped in this patch which were
explicitly build-tested), it also allows us to use this as a base for
the MSRV test, and preempt us from failing it whenever a (transitive!)
dependency bumps its MSRV beyond ours in a *semver-compatible* release.

* Elide redundant `impl` block lifetimes following stricter Rust 1.83 lint

Rust now points out that `impl<'a> (Trait for) Struct<'a>` is
superfluous whenever `'a` is not used anywhere else in the `impl` block.

.github/workflows/ci.yml

@@ -21,6 +21,20 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # Downgrade all dependencies to their minimum version, both to ensure our
+      # minimum version bounds are correct and buildable, as well as to satisfy
+      # our MSRV check when arbitrary dependencies bump their MSRV beyond our
+      # MSRV in a patch-release.
+      # This implies that downstream consumers can only rely on our MSRV when
+      # downgrading various (transitive) dependencies.
+      - uses: hecrj/setup-rust-action@v2
+        with:
+          rust-version: nightly
+        if: ${{ matrix.rust-version != 'stable' }}
+      - name: Downgrade dependencies
+        run: cargo +nightly generate-lockfile -Zminimal-versions
+        if: ${{ matrix.rust-version != 'stable' }}
+
       - uses: hecrj/setup-rust-action@v2
         with:
           rust-version: ${{ matrix.rust-version }}
@@ -87,13 +101,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
-          components: rustfmt
-
       - name: Format
         run: cargo fmt --all -- --check
         working-directory: android-activity