all: update to Go 1.20, use strings.CutPrefix/Suffix instead of our fork

Updates #7123
Updates #5309

Change-Id: I90bcd87a2fb85a91834a0dd4be6e03db08438672
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/static-analysis.yml

@@ -45,14 +45,17 @@ jobs:
   vet:
     runs-on: ubuntu-latest
     steps:
+    - name: Check out code
+      uses: actions/checkout@v3
+
     - name: Set up Go
       uses: actions/setup-go@v3
       with:
-        go-version: 1.19
-    - name: Check out code
-      uses: actions/checkout@v3
+        go-version-file: go.mod
+
     - name: Run go vet
       run: go vet ./...
+
     - uses: k0kubun/action-slack@v2.0.0
       with:
         payload: |
@@ -79,13 +82,14 @@ jobs:
             goarch: 386
 
     steps:
+    - name: Check out code
+      uses: actions/checkout@v3
+
     - name: Set up Go
       uses: actions/setup-go@v3
       with:
-        go-version: 1.19
+        go-version-file: go.mod
 
-    - name: Check out code
-      uses: actions/checkout@v3
 
     - name: Install staticcheck
       run: "GOBIN=~/.local/bin go install honnef.co/go/tools/cmd/staticcheck"