go: change the Go version support policy, drop one version

go.md

@@ -4,17 +4,18 @@
 
 Upon the release of a new version of Go, all repositories are updated
 according to the following conventions:
-* The minimum supported version of Go is set to two versions back from 
-  the previous one. For example, upon the release of Go 1.22, the minimum 
-  version becomes 1.20.
+* The minimum supported version of Go is set to one version back from
+  the current one. For example, upon the release of Go 1.22, the minimum
+  version becomes 1.21 (just like Go itself, release 1.22 makes 1.20
+  unsupported).
 * All dependency packages are updated (sometimes, according to project 
   requirements, some deprecated package methods may be updated as well.)
 * New Go version features are adapted where they make sense or can improve 
   performance.
 * Go versions in GitHub workflows are also updated as follows:
 
-  1. Testing is performed on the last three versions of Go. In the example
-  mentioned above, this would be `[ '1.20', '1.21', '1.22' ]`.
+  1. Testing is performed for the last two versions of Go. In the example
+  mentioned above, this would be `[ '1.21', '1.22' ]`.
   2. For build and cover jobs, the latest version should be used. In the 
   example mentioned above, this would be 1.22. 
   3. If there are new versions of GitHub actions, they can also be updated