Deprecate miner namespace support and tests

[kclowes]

What was wrong?

Geth removed support for their miner API since it's no longer needed after The Merge ™️ . See: https://geth.ethereum.org/docs/interacting-with-geth/rpc/ns-miner

How can it be fixed?

Add deprecation warnings to the methods within the miner namespace, and change the tests to check for the deprecation warnings. We can then either remove the methods as Geth stops supporting these, or wait until v7 to remove.

[fselmo]

closed by #3236 (v7) and #3238 (v6)

[molecula451]

what about chains that do still support "miner"