all: start suggesting Go 1.21 and testing on it

Also note that the first release is now 1.21.0,
so we no longer need to use the awkward 1.21.x notation in warnings.

.github/workflows/test.yml

@@ -25,7 +25,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.20.x]
+        go-version: [1.20.x, 1.21.x]
         os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps:
@@ -53,17 +53,17 @@ jobs:
     # Static checks from this point forward. Only run on one Go version and on
     # linux, since it's the fastest platform, and the tools behave the same.
     - name: Test third-party project builds
-      if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x'
+      if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.21.x'
       run: |
         go install
         ./scripts/check-third-party.sh
-    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x'
+    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.21.x'
       run: ./scripts/crlf-test.sh
-    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x'
+    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.21.x'
       run: diff <(echo -n) <(gofmt -d .)
-    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x'
+    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.21.x'
       run: go vet ./...
-    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x'
+    - if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.21.x'
       uses: dominikh/staticcheck-action@v1
       with:
         version: "2023.1.3"
@@ -81,11 +81,12 @@ jobs:
     - uses: actions/checkout@v3
     - uses: actions/setup-go@v4
       with:
-        go-version: 1.20.x
+        go-version: 1.21.x
         cache: false
     - run: go test -short ./...
 
   test-gotip:
+    if: false # let tip for 1.22 settle first
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3