Replace race warning confidences with semantic information

[sim642]

#1564 (comment) highlighted that race warning confidence values are rather arbitrary and not meaningful, especially when not looking how they're computed.

In GobCon on 2024-09-17 this was discussed and the idea would be to remove the numerical values. Instead, race warning accesses could have more semantic flags attached to them to convey the information that currently goes into the confidence computation:

• Direct access/pointer access/reachability access.
• Access via unknown pointer.
• ...