clean up to start v0.9 dev

[chicco785]

Proposed changes

• fix Remove CrateDB 3.x support #423
• fix Remove ThinkDB and InfluxDB experimental support #425
• fix Clarification on CrateDB resiliency and wait_for_active_shards = 1 #409
• other fixes eventually

Types of changes

What types of changes does your code introduce to the project: Put
an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after
creating the PR. If you're unsure about any of them, don't hesitate to
ask. We're here to help! This is simply a reminder of what we are going
to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• I have added tests that prove my fix is effective or that my
  feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in
  downstream modules

Further comments

N/A

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️

[c0c0n3]

shouldn't we leave this to @fisuda? not sure if he also mentioned influx db in the japanese docs?

[c0c0n3]

@chicco785 good stuff!! I only have one question about the Japanese docs, see my comment above.