docs: link to single source of truth for public pgp keys (aws#21541)

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### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

MANUAL_INSTALLATION.md

@@ -68,7 +68,7 @@ its signature against our public signing key. To do so, you need
 the following things:
 
 * [GNU Privacy Guard](https://gnupg.org/) needs to be installed.
-* Download our public key: https://s3.amazonaws.com/aws-cdk-beta/cdk-team.asc
+* Download our public key: https://docs.aws.amazon.com/cdk/v2/guide/pgp-keys.html
 * Make sure you have downloaded both `aws-cdk-x.y.z.zip`
   and `aws-cdk-x.y.z.zip.sig`.