ActionsSecret: 'plaintext_value' forces delete and create instead of inplace update

@AtzeDeVries @lacroi-m-insta
Using [plaintextValueSecretRef ](https://marketplace.upbound.io/providers/crossplane-contrib/provider-upjet-github/v0.18.0/resources/actions.github.upbound.io/ActionsSecret/v1alpha1#doc:spec-forProvider-plaintextValueSecretRef) causes the provider to detect diffs on the `plaintext_value` field which in turn cause Crossplane to try replacing the whole ActionSecret resource instead of updating its value, see: https://github.com/integrations/terraform-provider-github/issues/2518

Also note that there is no plainTextValue field available in the resource even if it is in the [terraform equivalent](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/actions_secret), so I can't use `initProvider` to ignore this field