chore(deps): update dependency go to v1.26.3 (cloudnative-pg#10647)

Bumps the `go` directive in `go.mod` to 1.26.3 (security release,
2026-05-07).

Pre-empts Renovate, which hasn't surfaced 1.26.3 yet because the hosted
datasource cache is still serving 1.26.2. Will be cherry-picked to
release branches.

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
(cherry picked from commit 91335f3)

Author: mnencia

go.mod

@@ -1,6 +1,6 @@
 module github.com/cloudnative-pg/cloudnative-pg
 
-go 1.26.2
+go 1.26.3
 
 require (
 	github.com/DATA-DOG/go-sqlmock v1.5.2