remove golang 1.18 due to atomic.Int64

Author: lzakharov

.github/workflows/doctests.yaml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [ "1.18", "1.19", "1.20", "1.21" ]
+        go-version: [ "1.19", "1.20", "1.21" ]
 
     steps:
       - name: Set up ${{ matrix.go-version }}