Switch stable box from alma to centos9 in 99-local

[chris1984]

We are using Centos9 for the stable boxes in vagrant cloud, so using alma would cause it to break.

Switching the name, we also are only publishing el9 to save resources and overhead.

[ekohl]

In #1826 and #1832 @archanaserver is also updating docs. Perhaps you could collaborate on making that consistent.

[nadjaheitmann]

Tested your PR and box starts up perfectly fine :)

[wbclark]

@chris1984 @archanaserver @ekohl How about we update docs/stable_boxes.md here and then #1826 can rebase?

[ekohl]

@wbclark that would be fine by me because I think we should get into a habit of updating the docs as part of the code change.

[wbclark]

I agree with @ekohl, this practice makes it easier to navigate the git history as well. What do you say, @chris1984 ?