Support Go 1.23+ only (#3711)

* Remove toolchain in go.mod

* Drop Go 1.21 and 1.22 from test workflow

Author: tchssk

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        go: ['1.21', '1.22', '1.23', '1.24']
+        go: ['1.23', '1.24']
         os: ['ubuntu-latest', 'windows-latest']
     runs-on: ${{ matrix.os }}
 

go.mod

@@ -2,8 +2,6 @@ module goa.design/goa/v3
 
 go 1.23.0
 
-toolchain go1.23.6
-
 require (
 	github.com/dimfeld/httppath v0.0.0-20170720192232-ee938bf73598
 	github.com/getkin/kin-openapi v0.132.0