feat: drop support for Go 1.15 (#145)

And add support for Go 1.18.

.github/update_go.sh

@@ -0,0 +1,37 @@
+# Copyright 2022 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# update_go.sh updates all GitHub Action workflows to use the provided Go
+# version.
+
+usage() {
+    echo "Missing Go version."
+    echo "usage: .github/update_go.sh <version>"
+    echo "  Example:"
+    echo "    .github/update_go.sh 1.18"
+}
+
+if [ -z "$1" ]
+  then
+    usage
+fi
+
+# Replace all usages of "go-version: x.xx" with the provided version.
+grep -lr "go-version: \"" .github/workflows | xargs \
+    sed -i "s/go-version: \".*\"/go-version: \"$1\"/g"
+
+# Replace the matrix of Go versions with the last two versions and the provided
+# version.
+grep -lr "go-version: \[" .github | xargs \
+    sed -i "s/\[\(.*\), \(.*\), \(.*\)\]/\[\2, \3, $1\]/"

.github/workflows/cover.yaml

@@ -22,7 +22,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v3
         with:
-          go-version: "1.17"
+          go-version: "1.18"
 
       - name: Checkout base branch
         uses: actions/checkout@v3

.github/workflows/lint.yaml

@@ -23,9 +23,9 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v3
         with:
-          go-version: "1.17"
+          go-version: "1.18"
       - name: Install goimports
-        run: go get golang.org/x/tools/cmd/goimports
+        run: go install golang.org/x/tools/cmd/goimports@latest
       - name: Checkout code
         uses: actions/checkout@v3
       - run: goimports -w .

.github/workflows/tests.yaml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [1.15, 1.16, 1.17]
+        go-version: [1.16, 1.17, 1.18]
     steps:
       - name: Setup Go ${{ matrix.go-version }}
         uses: actions/setup-go@v3