Pin @aws-sdk/client-s3 to 3.621.0 for Sample App to support Node.js 14 (#367)

*Issue description:*
Main Builds for ADOT JS are failing recently:
https://github.com/aws-observability/aws-otel-js-instrumentation/actions/workflows/main-build.yml

AWS SDK stopped supporting Node 14 recently
- https://github.com/aws/aws-sdk-js-v3/blame/main/README.md#L603-L606
-
https://github.com/aws/aws-sdk-js-v3/pull/6775/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5

We use ^3.621.0 [in the sample
app](https://github.com/aws-observability/aws-application-signals-test-framework/blob/main/sample-apps/node/frontend-service/package.json#L13)
used for testing, I tested the sample app uses a version that does not
work in Node 14 after `npm install`.

*Description of changes:*
- Pin @aws-sdk/client-s3 to 3.621.0 in JS Sample App to support Node.js
14

In the future, we can revert this change after Node.js 14 support is
dropped.

*Rollback procedure:*
Revert this PR, which is a safe change.

*Testing:*
- Sample app does not start before this change, sample app starts after
this change via Node 14

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

Author: jj22ee