fix(config): default S3 endpoint with region (#7807)

`AWS_ENDPOINT` defaults to an S3 endpoint hard-coded to ap-southeast-1.
This affects deployment scenarios to other AWS regions, unless
`AWS_ENDPOINT` is specified to line up with `AWS_REGION`.

It can be argued that the default for `AWS_ENDPOINT` should be driven
by the specific region that FormSG is deployed to, so accordingly, at
runtime, change `loadBucketS3UrlSchema()` so that the default endpoint
for S3 buckets reflects the region where FormSG is currently deployed.

src/app/config/schema.ts

@@ -502,7 +502,7 @@ export const loadS3BucketUrlSchema = ({
       doc: 'Endpoint for S3 buckets',
       format: (val) =>
         validateS3BucketUrl(val, { isDev, hasTrailingSlash: false, region }),
-      default: 'https://s3.ap-southeast-1.amazonaws.com', // NOTE NO TRAILING / AT THE END OF THIS URL!
+      default: `https://s3.${region}.amazonaws.com`, // NOTE NO TRAILING / AT THE END OF THIS URL!
       env: 'AWS_ENDPOINT',
     },
     attachmentBucketUrl: {