Fix sitemap 404

[tharders]

The sitemap.xml which is mentioned in the robots.txt results in a 404
https://amp.dev/sitemap.xml
Also the grow generated sitemap is not served.
https://amp.dev/sitemap_grow.xml

This PR fixes those issues.

[sebastianbenz]

Oh wow - that's bad! Good that you discovered this!

One comment.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of all the commit author(s), set the cla label to yes (if enabled on your project), and then merge this pull request when appropriate.

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[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[sebastianbenz]

Looks good! One nit!