chore: modernize Fly setup

[MichaelDeBoey]

Just like I did in https://github.com/remix-run/indie-stack/pull/253 & https://github.com/remix-run/blues-stack/pull/201

I took inspiration from @kentcdodds' Epic Stack + workshop repos

• https://github.com/epicweb-dev/epic-stack/commit/b723ea817ec0f5ca71a088334c4440ff4e9702ee
• https://github.com/epicweb-dev/web-auth/commit/4bfb376f17cb9afcf0460536d388b1052d2e4df4
• https://github.com/epicweb-dev/web-forms/commit/9ce223b5ec4c81d827afa047e908e0162f295881
• https://github.com/epicweb-dev/data-modeling/commit/7fd3f38663423d960ff62a849b46912da361f924
• https://github.com/epicweb-dev/full-stack-testing/commit/d220efc1d4c0e153ba1919228c6ffe5fcb484a5c
• https://github.com/epicweb-dev/full-stack-foundations/commit/defdda31785b19615b47130b446e3a7bfbef6266