ci: use "oldstable" and "stable" Go versions

In addition to the one from go.mod (currently 1.21).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Author: kolyshkin

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
       - uses: actions/setup-go@v6
         with:
-          go-version: 1.x # latest
+          go-version: stable
       - uses: actions/checkout@v5
       - uses: golangci/golangci-lint-action@v8
         with:
@@ -44,7 +44,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [1.21.x, 1.22.x, 1.23.x]
+        go-version: [1.21.x, oldstable, stable]
         race: ["-race", ""]
 
     steps: