[ECR Public] [request]: increase character limit for About and Usage sections

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**Tell us about your request**
I'd like to see an increase in character count for 'About' and 'Usage' sections from `10240` to `25000` (or higher!)

**Which service(s) is this request for?**
ECR Public

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Our team is trying to keep GitHub ReadMes in sync with ECR Public Repo 'About' and 'Usage' sections; however, it is not possible to upload the entire ReadMe due to char limit being too low.

**Are you currently working around this issue?**
Yes, I have a PR out https://github.com/aws/amazon-ec2-metadata-mock/pull/117 to truncate the ReadMe based on char limits

**Additional context**
We also sync the ReadMes to DockerHub and have not run into any issues due to their char limit being set to 25000.

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing issue #1219 and the ECR Public About and Usage character-limit behavior; no repository file, test, or entry point is identified. Done means the limit increases from 10,240 to 25,000 or higher so the full README can sync without truncation.

Written by the indexing model from the issue text.