Add <H3> headers for Attributes section in modal

fixes MAT-385
flag=none

Test Plan:
 - Navigate to a RCE instance
 - Click on 'Upload Image'
 - Inspect the modal open and check
 'Attributes'. Now the title of
 this section is closed in a <H3>
 tag
 - Test with VoiceOver and navigate among
 the modal elements with the keyboard.
 'Attributes' is now mentioned or included
 in the browsing

Change-Id: I0708115da777c832e27c34a9d55b05a5adb533b0
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/286200
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jake Oeding <jake.oeding@instructure.com>
QA-Review: Jake Oeding <jake.oeding@instructure.com>
Product-Review: David Lyons <lyons@instructure.com>

packages/canvas-rce/src/rce/plugins/shared/Upload/UploadFileModal.js

@@ -250,7 +250,7 @@ const UploadFileModal = React.forwardRef(
                     <ToggleDetails
                       defaultExpanded
                       summary={
-                        <Text size="x-large">{formatMessage('Usage Rights (required)')}</Text>
+                        <Heading level="h3">{formatMessage('Usage Rights (required)')}</Heading>
                       }
                     >
                       <UsageRightsSelectBox
@@ -273,7 +273,7 @@ const UploadFileModal = React.forwardRef(
                   >
                     <ToggleDetails
                       defaultExpanded={!requiresUsageRights}
-                      summary={<Text size="x-large">{formatMessage('Attributes')}</Text>}
+                      summary={<Heading level="h3">{formatMessage('Attributes')}</Heading>}
                     >
                       <ImageOptionsForm
                         id="upload-file-form"