build: use Go 1.21 as a minimum version

In go.mod, starting from 1.21, go version can be X.Y.Z, which is not
supported by older versions. As some dependencies use this format, we
have to bump the version. The alternative is to pin tools' versions to
older versions instead of latest.

Author: vincentbernat

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         go-version:
-          - 1.18
+          - 1.21
           - oldstable
           - stable
         os:

README.md

@@ -24,7 +24,7 @@ can checkout tag [v0.6][] if you need compatibility up to Go 1.11.
 
 [v0.6]: https://github.com/vincentbernat/hellogopher/tree/v0.6
 
-Some tools now require more recent versions of Go (1.18+), but it would be
+Some tools now require more recent versions of Go (1.21+), but it would be
 possible to pin them to older versions.
 
 On first build, you need to run `go mod init PROJECTNAME`.