Test with Go 1.20 (#295)

Each major Go release is supported until there are two newer major
releases. Removing testing with Go 1.18 because `staticcheck` has 
conflicting versions that don't work either on 1.18 (v0.4.0) or 1.20 (v0.3.3).

.github/workflows/push.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        goVersion: [1.18.x, 1.19.x]
+        goVersion: [1.19.x, 1.20.x]
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -31,8 +31,6 @@ jobs:
         run: |
           echo "GOPATH=$(go env GOPATH)" >> $GITHUB_ENV
           echo "$(go env GOPATH)/bin" >> $GITHUB_PATH
-          go install gotest.tools/gotestsum@latest
-          go install honnef.co/go/tools/cmd/staticcheck@latest
       
       - name: Pull external libraries
         run: make vendor

Makefile

@@ -12,7 +12,7 @@ fmt:
 
 lint: vendor
 	@echo "✓ Linting source code with https://staticcheck.io/ ..."
-	@go run honnef.co/go/tools/cmd/staticcheck@latest ./...
+	@go run honnef.co/go/tools/cmd/staticcheck@v0.4.0 ./...
 
 test: lint
 	@echo "✓ Running tests ..."