Require a patched Go toolchain

go.mod required go 1.26.0, whose standard library has known
vulnerabilities reachable from this module according to govulncheck.
Raise the minimum to the latest 1.26 patch release.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

go.mod

@@ -1,6 +1,6 @@
 module github.com/datekeys/datekeys-go
 
-go 1.26.0
+go 1.26.8
 
 require (
 	filippo.io/age v1.3.2