Bug 1801738 - Use --enable-project instead of --enable-application. r=firefox-build-system-reviewers,geckoview-reviewers,calu,ahochheiden

--enable-application is the historic flag, and --enable-project was
added to handle the non-application things we build, while encompassing
the meaning of --enable-application. --enable-project has been preferred
for a while and we should reflect that more consistently in mozconfigs,
documentation, etc.

Differential Revision: https://phabricator.services.mozilla.com/D162625