go.mod: update Go toolchain base to 1.21

To access the "maps" library package.
diff --git a/go.mod b/go.mod
index feaecad..c02809b 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module bitbucket.org/creachadair/stringset
 
-go 1.20
+go 1.21
 
 require honnef.co/go/tools v0.4.7
 
diff --git a/go.sum b/go.sum
index 631c929..93c48ff 100644
--- a/go.sum
+++ b/go.sum
@@ -5,6 +5,7 @@
 golang.org/x/mod v0.15.0 h1:SernR4v+D55NyBH2QiEQrlBAnj1ECL6AGrA5+dPaMY8=
 golang.org/x/mod v0.15.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
 golang.org/x/sync v0.6.0 h1:5BMeUDZ7vkXGfEr1x9B4bRcTH4lpkTkpdh0T/J+qjbQ=
+golang.org/x/sync v0.6.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
 golang.org/x/tools v0.18.0 h1:k8NLag8AGHnn+PHbl7g43CtqZAwG60vZkLqgyZgIHgQ=
 golang.org/x/tools v0.18.0/go.mod h1:GL7B4CwcLLeo59yx/9UWWuNOW1n3VZ4f5axWfML7Lcg=
 honnef.co/go/tools v0.4.7 h1:9MDAWxMoSnB6QoSqiVr7P5mtkT9pOc1kSxchzPCnqJs=