docs: update changelog and release docs for Gaia v21

[sainoe]

TODO:

• Check that ICS dependency targets the v6.2.x release
• Add ICS bump to changelog entries
• Update changelog

Closes: #XXXX

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct docs: prefix in the PR title
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• Confirmed the correct docs: prefix in the PR title
• Confirmed all author checklist items have been addressed
• Confirmed that this PR only changes documentation
• Reviewed content for consistency
• Reviewed content for thoroughness
• Reviewed content for spelling and grammar
• Tested instructions (if applicable)

[mpoke]

See my comments.

[mpoke]

Mainnet is running v20.0.0 and not v20.0.1. See https://github.com/cosmos/gaia/releases/tag/v20.0.1