Drop support for Go < 1.18

In the next patch we will need to use generics, which are only available
starting from Go 1.18 . That Go 1.18 was released in spring of 2022 -
almost 2.5 years ago - and is already EOL now. The older releases became
EOL even earlier compared to Go 1.18 and are generally no longer
supported by Go upstream. Dropping support for Go < 1.18 remains support
for Go 1.18 - 1.23, i.e. 6 releases 3 of which are already in end of
life state. So dropping support for Go < 1.18 should be, hopefully, ok.

.github/workflows/ci.yml

@@ -11,12 +11,6 @@ jobs:
     strategy:
       matrix:
         go:
-          - "1.12.x"
-          - "1.13.x"
-          - "1.14.x"
-          - "1.15.x"
-          - "1.16.x"
-          - "1.17.x"
           - "1.18.x"
           - "1.19.x"
           - "1.20.x"

go.mod

 module github.com/kisielk/og-rek
 
-go 1.12
+go 1.18