docs: moved jaeger-postgresql to remote plugin list

[robbert229]

Which problem is this PR solving?

• Updating documentation to reflect current state of jaeger-postgresql plugin. The plugin currently uses the remote storage interface.

Description of the changes

• Moves jaeger-postgresql to the remote storage backend plugin list.

How was this change tested?

• N/A

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[yurishkuro]

Thanks for helping to keep the docs up to date!