CI: Check MSRV in CI (#37152)

As previously proposed on zulip and discussed in the coordination
meeting, add a check to CI to see if
servo still compiles with our minimum supported Rust version.
To avoid requiring changes, we define our MSRV as the current version we
are using now (1.85.0).

This does not prevent us from updating the default compiler version,
which we should still do, to
get benefits like faster compile times, newer lints and making sure
crown stays up-to-date.

We simply test that libservo compiles in CI, since libservo (and
dependencies) is what embedders would care about. We also don't need
mach (or bootstrap!) for this, so we just use cargo build.

Testing: This PR adds a CI test. [`./mach try windows-build-libservo
linux-build-libservo
mac-build-libservo`](https://github.com/jschwe/servo/actions/runs/16901171766)

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

.github/actions/parse_msrv/action.yml

@@ -0,0 +1,21 @@
+name: Parse rust-version from libservo
+description: "Parse rust-version from libservo. Requires cargo-metadata."
+outputs:
+  rust_version:
+    description: "Minimum rust-version defined by libservo"
+    value: ${{ steps.parse_version.outputs.LIBSERVO_RUST_VERSION }}
+
+runs:
+  using: "composite"
+  steps:
+    - name: Parse libservo version
+      id: parse_version
+      shell: bash
+      run:  |
+        msrv=$(cargo metadata --no-deps --format-version 1 | jq -r '.packages[] | select(.name == "libservo") | .rust_version')
+        if [[ "${msrv}" == "null" ]]; then
+          echo "Failed to determine MSRV of libservo"
+          exit 1
+        fi
+        echo "libservo has a minimum supported Rust version of ${msrv}."
+        echo "LIBSERVO_RUST_VERSION=${msrv}" >> "$GITHUB_OUTPUT"