[V4] Simplify the set of controls around downloading artifacts

Is your feature request related to a problem? Please describe.

There exist many different controls around content downloading that all accomplish very similar, but not exactly the same things.

MAX_CONCURRENT_CONTENT
download_concurrency (on Remote)
rate_limit (on Remote)
SYNC_MAX_IN_FLIGHT_ITEMS and SYNC_MAX_IN_FLIGHT_MB (recently added)
Batch size of the download stage

Having all of these knobs is confusing and makes the code much more complicated than it needs to be.
We should audit all the different ways in which these overlap and try to simplify if we can.

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Additional context
derived from this conversation: https://github.com/pulp/pulpcore/pull/7649#discussion_r3209269413

Start by auditing MAX_CONCURRENT_CONTENT, Remote.download_concurrency, Remote.rate_limit, SYNC_MAX_IN_FLIGHT_ITEMS, SYNC_MAX_IN_FLIGHT_MB, and the download-stage batch size. Read the linked pull request discussion for context and trace where each control is applied. Done means the overlapping controls have a simpler, agreed design with their responsibilities clearly defined.