chore(deps): update go toolchain directive to v1.26.3 (#895)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [go](https://go.dev/)
([source](https://redirect.github.com/golang/go)) | toolchain | patch |
`1.26.2` → `1.26.3` |

---

### Release Notes

<details>
<summary>golang/go (go)</summary>

###
[`v1.26.3`](https://redirect.github.com/golang/go/compare/go1.26.2...go1.26.3)

</details>

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go.mod

@@ -2,7 +2,7 @@ module github.com/cloudnative-pg/plugin-barman-cloud
 
 go 1.26.0
 
-toolchain go1.26.2
+toolchain go1.26.3
 
 require (
 	github.com/cert-manager/cert-manager v1.20.2