[server] Move the server related gitattributes within that folder (#1106)

I mistakenly assumed that the gitattributes file can only be at the top
level. However, this doesn't seem to be the case - I noticed this in a
recent PR - https://github.com/ente-io/ente/pull/1104/

And it is indeed documented:

> When deciding what attributes are assigned to a path, Git ....
.gitattributes file in the same directory as the path in question. ..
>
> - https://git-scm.com/docs/gitattributes

Ref:
https://stackoverflow.com/questions/71784062/can-we-have-multiple-gitattributes-files-in-different-repo-subfolders