Merge Pull Request Modal: Informative image is missing alt text

Issue Summary

The informative image showing the branches to be merged in the Merge pull request modal has a missing alt text.

Steps to Reproduce

1. Launch the URL
2. In the main content in the Top section navigate to the "Merge" button.
3. After activating the Merge button a "Merge pull request" modal will open, in which navigate to the image showing branches to merge.

Screenshot

Actual Results

In the Merge pull request modal dialog, alt text was missing for the informative image which shows the merging of two branches, i.e. which branch will merge with which branch.

Alternate text is helpful for users with visual disabilities, learning disabilities, users browsing with text browsers etc. to understand the information conveyed using images. 

Expected Results

The following points could be used to make the image accessible.

1. Use the alt attribute on the <img> element to convey the intent, purpose, or meaning of the image.

eg.

<img alt="Merge Test1.2 into Test1.1" class="branches-diagram" src="/s/c355b0a0-332e-4486-a290-12c9cdb7bd8c/_/download/resources/com.atlassian.bitbucket.server.bitbucket-frontend:assets-c355b0a0-332e-4486-a290-12c9cdb7bd8c/e7b6cd354c7c7e230526efe80e4bdbf9.svg">

2. Use the aria-label attribute to convey the intent, purpose, or meaning of the image.

Workaround

Currently, there is no known workaround for this behaviour. A workaround will be added here when available

Bug Ref: 735489