Replace DispatchQueue.Sync with Actors

There are a number of places in the app that uses DispatchQueue.sync to prevent multi threaded race conditions that seem like good candidates to switch to Actors.

Example: https://github.com/Automattic/pocket-casts-ios/blob/402e8281c6668ef0cd679701057d5e21fb2b612e/podcasts/DownloadProgressManager.swift#L16

Start with the DispatchQueue.sync usage in podcasts/DownloadProgressManager.swift at the linked example, then identify the other locations in the app that use the same pattern. Determine which usages are candidates for Actors and define completion as replacing the appropriate race-prevention patterns without regressions.

Written by the indexing model from the issue text.