Add dedicated cluster clarity

[lzdanski]

Issue Type

Inaccurate, misleading, or out-of-date information

Links to Affected Docs

• https://docs.astronomer.io/astro/create-dedicated-cluster
• https://docs.astronomer.io/astro/connect-aws

The Issue or Context

Add to our docs that the minimum size for a VPC CIDR is /20

Fix or Proposed Change

No response

Additional Notes

• Slack conversation

Required Reviewers

No response