fix(deps): update module github.com/cert-manager/cert-manager to v1.21.1

| datasource | package                              | from    | to      |
| ---------- | ------------------------------------ | ------- | ------- |
| go         | github.com/cert-manager/cert-manager | v1.21.0 | v1.21.1 |


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

go.mod

@@ -3,7 +3,7 @@ module github.com/cloudnative-pg/plugin-barman-cloud
 go 1.26.4
 
 require (
-	github.com/cert-manager/cert-manager v1.21.0
+	github.com/cert-manager/cert-manager v1.21.1
 	github.com/cloudnative-pg/api v1.30.0
 	github.com/cloudnative-pg/barman-cloud v0.5.2-0.20260720143032-950b0f57e122
 	github.com/cloudnative-pg/cloudnative-pg v1.30.0