Update maintenance mode message to justify hint

[trivikr]

As per AWS SDKs and Tools maintenance policy Phase 2, a public announcement needs to be made before an SDK version is put into maintenance mode. This announcement gives users at least six months to upgrade, and calls out the date for Phase 3. This is usually done through blog post, documentation and deprecation warnings.

In #4345, we added a hint about our plans to put v2 into maintenance mode with the following warning:

The AWS SDK for JavaScript (v2) will be put into maintenance mode in 2023.

Please migrate your code to use AWS SDK for JavaScript (v3).
For more information, check the migration guide at https://a.co/7PzMCcy

However, users are confusing it with the official announcement which is not made yet.

This PR uses the statement from README instead:

We are formalizing our plans to enter AWS SDK for JavaScript (v2) into maintenance mode in 2023.

Please migrate your code to use AWS SDK for JavaScript (v3).
For more information, check the migration guide at https://a.co/7PzMCcy

Checklist

• npm run test passes

[shellscape]

@trivikr the CDK team has a lovely way to disable warnings. any chance you could get with them to replicate that within aws-sdk so we can hide this message instead of seeing it on every run?