Improve "get mined blocks" query performance

[vbaranov]

Motivation

https://forum.poa.network/t/querying-core-poa-network-blockchain-via-rpc-graphql/3591

I’m trying to pull data from POA Core blockchain for analytics (e.g. blocks mined and list of transactions from specific POA address) via RPC and it’s timing out even for a single page with an offset of 1 (i.e. asking for just one item).

Changelog

Remove grouping by number, timestamp

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools