Improve Bazel CI (#8502)

* Update Buildkite Bazel CI

Restructure presubmit.yml to support matrix.

* Remove testing Ubuntu 18.04

The available LLVM 6.0 is too old to support std::filesystem.

* Add testing in Ubuntu 22.04

* Use Bazel version 6.5.0 in integration test

.bazelci/presubmit.yml

@@ -1,22 +1,12 @@
 ---
 buildifier: latest
-bazel: 6.4.0
-platforms:
-  ubuntu1804:
-    environment:
-      CC: clang
-      SWIFT_VERSION: "5.8"
-      SWIFT_HOME: "$HOME/swift-$SWIFT_VERSION"
-      PATH: "$PATH:$SWIFT_HOME/usr/bin"
-    shell_commands:
-      - "echo --- Downloading and extracting Swift $SWIFT_VERSION to $SWIFT_HOME"
-      - "mkdir $SWIFT_HOME"
-      - "curl https://download.swift.org/swift-${SWIFT_VERSION}-release/ubuntu1804/swift-${SWIFT_VERSION}-RELEASE/swift-${SWIFT_VERSION}-RELEASE-ubuntu18.04.tar.gz | tar xvz --strip-components=1 -C $SWIFT_HOME"
-    build_targets:
-    - "//..."
-    test_targets:
-    - "//..."
-  ubuntu2004:
+matrix:
+  bazel:
+    - 6.x
+tasks:
+  verify_ubuntu2004:
+    platform: ubuntu2004
+    bazel: ${{ bazel }}
     environment:
       CC: clang
       SWIFT_VERSION: "5.8"
@@ -30,7 +20,25 @@ platforms:
     - "//..."
     test_targets:
     - "//..."
-  macos:
+  verify_ubuntu2204:
+    platform: ubuntu2204
+    bazel: ${{ bazel }}
+    environment:
+      CC: clang
+      SWIFT_VERSION: "5.8"
+      SWIFT_HOME: "$HOME/swift-$SWIFT_VERSION"
+      PATH: "$PATH:$SWIFT_HOME/usr/bin"
+    shell_commands:
+      - "echo --- Downloading and extracting Swift $SWIFT_VERSION to $SWIFT_HOME"
+      - "mkdir $SWIFT_HOME"
+      - "curl https://download.swift.org/swift-${SWIFT_VERSION}-release/ubuntu2204/swift-${SWIFT_VERSION}-RELEASE/swift-${SWIFT_VERSION}-RELEASE-ubuntu22.04.tar.gz | tar xvz --strip-components=1 -C $SWIFT_HOME"
+    build_targets:
+    - "//..."
+    test_targets:
+    - "//..."
+  verify_macos:
+    platform: macos
+    bazel: ${{ bazel }}
     xcode_version: "14.3"
     build_targets:
     - "//:flatbuffers"