fix: [sc-118962] Unable to Retrieve TLS Parameters from Kubernetes Secrets with the Postgres Collector

[nvanthao]

Story details: https://app.shortcut.com/replicated/story/118962

Changing from reading secret from StringData to Data field.

From K8S API doc

        // stringData allows specifying non-binary secret data in string form.
	// It is provided as a write-only input field for convenience.
	// All keys and values are merged into the data field on write, overwriting any existing values.
	// The stringData field is never output when reading from the API.
	// +k8s:conversion-gen=false
	// +optional
	StringData map[[string](https://pkg.go.dev/builtin#string)][string](https://pkg.go.dev/builtin#string) `json:"stringData,omitempty" protobuf:"bytes,4,rep,name=stringData"`

[diamonwiggins]

a comment.

[diamonwiggins]

lgtm