[Jetpack Social] Convert `_wpas_skip` metadata to `_wpas_skip_publicize`

Currently some of our clients are using _wpas_skip metadata to represent the sharing social accounts that were manually disabled by the user. Back-end is migrating to a new standard, using the key _wpas_skip_publicize. Android never supported the legacy way and from this PR started to support _wpas_skip_publicize, but we might get the legacy _wpas_skip metadata in PostModel that might've been added in another client. The purpose of this issue is to detect if the metadata contains any _wpas_skip and then migrate it to the new standard _wpas_skip_publicize.

Legacy key: _wpas_skip_<keyringConnectionID>
New key: _wpas_skip_publicize_<publicizeConnectionID>

When receiving a post with any legacy keys in the metadata, we should update it with the new format. It's very important to use the correct ID: legacy key used the keyring connection ID and the new key uses the publicize connection ID.

Discussion: p1690306257631299-slack-C05362VSXFU

Start at PostModel metadata handling and trace how keyring connection IDs map to publicize connection IDs. Verify that every wpas_skip entry is migrated to wpas_skip_publicize, while preserving the intended disabled social accounts.