Update go toolchain directive to v1.26.4 (#291)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

go.mod

@@ -2,7 +2,7 @@ module github.com/stackitcloud/gardener-extension-acl
 
 go 1.26.0
 
-toolchain go1.26.3
+toolchain go1.26.4
 
 require (
 	github.com/gardener/gardener v1.143.1