modfile: use new go version string format in error message

For golang/go#61888

Change-Id: If4056623471edb6fd99d45dcd8a0751d6a54ce1c
Reviewed-on: https://go-review.googlesource.com/c/mod/+/527897
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

modfile/rule.go

@@ -367,7 +367,7 @@ func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, a
 				}
 			}
 			if !fixed {
-				errorf("invalid go version '%s': must match format 1.23", args[0])
+				errorf("invalid go version '%s': must match format 1.23.0", args[0])
 				return
 			}
 		}
@@ -384,7 +384,7 @@ func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, a
 			errorf("toolchain directive expects exactly one argument")
 			return
 		} else if strict && !ToolchainRE.MatchString(args[0]) {
-			errorf("invalid toolchain version '%s': must match format go1.23 or local", args[0])
+			errorf("invalid toolchain version '%s': must match format go1.23.0 or local", args[0])
 			return
 		}
 		f.Toolchain = &Toolchain{Syntax: line}