Update Juju storage documentation for PostgreSQL 16

[a-velasco]

The juju-storage section is using the same storage type name (pgdata) as pgsql 14 although pgsql 16 has four different types of storage and none of them is named pgdata.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-8961.

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