Run tests on golang 1.21

And update README to mention that golang 1.20 is earliest tested for
upcomming 0.16.1 release

Author: Civil

.github/workflows/tests.yml

@@ -14,8 +14,8 @@ jobs:
     strategy:
       matrix:
         go:
-          - ^1.19
           - ^1.20
+          - ^1.21
           - ^1
     steps:
 

README.md

@@ -119,7 +119,7 @@ Golang compatibility matrix:
 | 1.12           | 0.12.6                           |
 | 1.16 / 1.17    | 0.15.6                           |
 | 1.18           | 0.16.0-patch2                    |
-| 1.19           | current                          |
+| 1.20           | 0.16.1                           |
 
 Overall rule of thumb is that carbonapi supports last 2 major go versions. E.x. at this moment Go 1.19 and 1.18 are supported.