Deprecate S3 CopySource parameter for CopyObject & UploadPartCopy

## Motivation and Context

In aws#2612 we recently
introduced replacement parameters for the S3 CopySource parameter.
However, we did not officially deprecate it since our codegen tooling
lacked support for deprecated members at the time. With
aws#2622 we have since added
support for deprecated members to both models and model modifications
via customization.config files.

Now that this feature is fully supported, we should deprecate the
CopySource parameter to increase visibility and improve discoverability
of the new parameters that are offered. This will make users more likely
to discover the new parameters, allowing them to simplify their
application code accordingly.

## Description

* Annotate the CopySource parameter as deprecated for both CopySource
and UploadPartCopy

Author: Bennett Lynch

.changes/next-release/documentation-AmazonS3-259552c.json

@@ -0,0 +1,6 @@
+{
+    "category": "Amazon S3", 
+    "contributor": "", 
+    "type": "documentation", 
+    "description": "Deprecate S3 CopySource parameter for CopyObject & UploadPartCopy"
+}

services/s3/src/main/resources/codegen-resources/customization.config

@@ -35,6 +35,10 @@
       ],
       "modify": [
         {
+          "CopySource": {
+            "deprecated": true,
+            "deprecatedMessage": "The {@code copySource} parameter has been deprecated in favor of the more user-friendly {@code sourceBucket}, {@code sourceKey}, and {@code sourceVersionId} parameters. The {@code copySource} parameter will remain fully functional, but it must not be used in conjunction with its replacement parameters."
+          },
           "Bucket": {
             "emitPropertyName": "DestinationBucket",
             "existingNameDeprecated": true
@@ -65,6 +69,10 @@
       ],
       "modify": [
         {
+          "CopySource": {
+            "deprecated": true,
+            "deprecatedMessage": "The {@code copySource} parameter has been deprecated in favor of the more user-friendly {@code sourceBucket}, {@code sourceKey}, and {@code sourceVersionId} parameters. The {@code copySource} parameter will remain fully functional, but it must not be used in conjunction with its replacement parameters."
+          },
           "Bucket": {
             "emitPropertyName": "DestinationBucket",
             "existingNameDeprecated": true