Miner should extend from the latest sortition in which it mined blocks, not the latest sortition it won

[brice-stacks]

This problem extends from the problem described in #6481:

• Miner A wins tenure N
• Miner A submits block commit confirming tenure N-1 before any of its blocks for tenure N have been approved
• Miner A mines 2 or more blocks in tenure N
• Miner A wins tenure N+1 with the block commit confirming N-1 (maybe it failed to RBF the block commit, or maybe it never tried for some reason)
• Miner A cannot mine a tenure change + coinbase in N+1 because it would be reorging its own blocks from tenure N
• With the fix from #6513, Miner A will now tenure extend from tenure N and continue mining
• In tenure N+2, the null miner wins, so Miner A should extend its tenure and continue mining
• Miner A attempts to mine from tenure N+1 instead of continuing tenure N, but these proposals are all rejected, since they are attempting to reorg