Programmatically opt-out of SqlClient's retrying behavior for OpenAsync in EF's database existence check

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SqlClient 5.1.2 added retrying behavior in the asynchronous OpenAsync; this already existed for sync Open, with a programmatic opt-out that EF uses when checking for a database's existence, but no such programmatic opt-out was introduced for the new async version.

The SqlClient plan is to revert the new async retry behavior for 5 (comment), and re-introduce it later in SqlClient 6 along with the opt-out. When that happens and when EF switches to SqlClient 6, we need to react and make use of that opt out.

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Note: the actual regression should be fixed by bumping our SqlClient dependency to the soon-to-be-released 5.1.6, which fully reverts the SqlClient change. This issue tracks implementing the opt-out when we upgrade 6.0.