Update LLVM version to 15.0 in CI workflows (sigp#4220)

The latest stable version (1.69.0) of Rust was released on 20 April and contains this change:
- [Update the minimum external LLVM to 14.](rust-lang/rust#107573)

This impacts some of our CI workflows (build and release-test-windows) that uses LLVM 13.0. This  PR updates the workflows to install LLVM 15.0.

**UPDATE**: Also updated `h2` to address [this issue](GHSA-f8vr-r385-rh5r)

Author: jimmygchen

.github/workflows/release.yml

@@ -73,7 +73,7 @@ jobs:
             - uses: KyleMayes/install-llvm-action@v1
               if: startsWith(matrix.arch, 'x86_64-windows')
               with:
-                version: "13.0"
+                version: "15.0"
                 directory: ${{ runner.temp }}/llvm
             - name: Set LIBCLANG_PATH
               if: startsWith(matrix.arch, 'x86_64-windows')

.github/workflows/test-suite.yml

@@ -83,7 +83,7 @@ jobs:
       run: choco install -y make
     - uses: KyleMayes/install-llvm-action@v1
       with:
-        version: "13.0"
+        version: "15.0"
         directory: ${{ runner.temp }}/llvm
     - name: Set LIBCLANG_PATH
       run: echo "LIBCLANG_PATH=$((gcm clang).source -replace "clang.exe")" >> $env:GITHUB_ENV