Remove toolchain directive from go.mod

This directive specified a newer version of the go toolchain from the go
runtime used, and newer than our minimum supported.

- Fixes #1571

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

.github/workflows/check.yml

@@ -62,7 +62,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ['1.23'] # ['1.21', '1.22', '1.23'] # https://github.com/warewulf/warewulf/issues/1571
+        go-version: ['1.21', '1.22', '1.23']
     steps:
       - name: Checkout Warewulf
         uses: actions/checkout@v4

go.mod

@@ -2,8 +2,6 @@ module github.com/warewulf/warewulf
 
 go 1.21.0
 
-toolchain go1.23.3
-
 require (
 	dario.cat/mergo v1.0.0
 	github.com/Masterminds/sprig/v3 v3.2.3