fix: use bor_getAuthor instead of block.miner to fetch block miner

[Raneet10]

Description

Earlier when fetching a block via getBlock returned the block producer address instead of 0x00... in the block.miner field. This change was reverted in maticnetwork/bor#631 . This led to breaking some assertions in EIP-1559 test script. This PR fixes that.

Changes

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• New test case for remote devnet

Checklist

• I have added at least 2 reviewer or the whole pos-v1 team
• I have added sufficient documentation in code
• I will be resolving comments - if any - by pushing each fix in a separate commit and linking the commit hash in the comment reply

Testing

• I have tested this code manually on local environment
• I have tested this code manually on remote devnet using express-cli
• I have tested this code manually on mumbai