Release tracking issue

This issue tracks release [2104240357000](https://github.com/ampproject/amphtml/releases/tag/2104240357000).

• Release 2104240357000 promoted to Experimental and Beta (opt-in) channels ()
• Release 2104240357000 promoted to Experimental and Beta (1% traffic) channels ()
• Release 2104240357000 promoted to Stable channel (May 5, 2021, 8:56:59 AM GMT-7)

See the release documentation for more information on the release process, including how to test changes in the Experimental channel.

If you find a bug in this build, please file an issue. If you believe the bug should be fixed in this build, follow the instructions in the cherry picks documentation.

/cc @ampproject/release-on-duty