wip: temporarily take MSRV 1.73

1.71 and 1.72 both hit an ICE building w/ aws-lc-rs HPKE suites. There's
probably a way to work around this/slash get it fixed upstream, but for
now let's just bump MSRV to get green CI for review.

Along the way, match the formatter to the MSRV. It had fallen behind.

.github/workflows/test.yaml

@@ -25,7 +25,7 @@ jobs:
           - nightly
           # MSRV - keep in sync with what rustls and rustls-platform-verifier
           # consider MSRV
-          - 1.71.0
+          - "1.73"
         os: [ ubuntu-latest ]
         # but only stable, clang, and aws-lc-rs on macos (slower platform)
         include:
@@ -256,7 +256,7 @@ jobs:
       - name: Install rust toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.67.1
+          toolchain: "1.73" # Matching MSRV
           components: rustfmt
       - name: Check Rust formatting
         run: cargo fmt --all -- --check