Fix go.mod for GitHub Actions compatibility

- Change go version from 1.24.0 to 1.21 (stable version)
- Remove toolchain directive (not supported in older Go versions)
- This fixes CI build failures with 'invalid go version' error

Author: yaronf

go.mod

@@ -1,8 +1,6 @@
 module github.com/yaronf/httpsign
 
-go 1.24.0
-
-toolchain go1.24.1
+go 1.21
 
 require (
 	github.com/andreyvit/diff v0.0.0-20170406064948-c7f18ee00883