DEV: Upgrade Uppy to v4 (#29397)

Key changes include:

- `@uppy/aws-s3-multipart` is now part of `@uppy/aws-s3`, and controlled with a boolean

- Some minor changes/renames to Uppy APIs

- Uppy has removed batch signing from their S3 multipart implementation. This commit implements a batching system outside of Uppy to avoid needing one-signing-request-per-part

- Reduces concurrent part uploads to 6, because S3 uses HTTP/1.1 and browsers limit concurrent connections to 6-per-host.

- Upstream drop-target implementation has changed slightly, so we now need `pointer-events: none` on the hover element

plugins/chat/test/javascripts/components/chat-composer-uploads-test.js

@@ -120,9 +120,8 @@ module("Discourse Chat | Component | chat-composer-uploads", function (hooks) {
     this.appEvents.on(
       `upload-mixin:chat-composer-uploader:upload-cancelled`,
       (fileId) => {
-        assert.strictEqual(
-          fileId.includes("uppy-avatar/"),
-          true,
+        assert.true(
+          fileId.includes("chat-composer-uploader-avatar/"),
           "upload was cancelled"
         );
         done();