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/assets/stylesheets/common/base-common.scss

@@ -26,6 +26,7 @@ html.ios-device.keyboard-visible body #main-outlet .full-page-chat {
   left: 0;
   background-color: rgba(0, 0, 0, 0.75);
   z-index: z("header");
+  pointer-events: none;
   &-content {
     width: max-content;
     display: flex;

plugins/chat/assets/stylesheets/common/chat-upload-drop-zone.scss

@@ -10,6 +10,7 @@
   justify-content: center;
   display: flex;
   background: rgba(var(--always-black-rgb), 0.85);
+  pointer-events: none;
 
   .uppy-is-drag-over & {
     visibility: visible;

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();