macOS deployments only work on the latest macOS version

[janhohenheim]

The issue is MACOSX_DEPLOYMENT_TARGET here: https://github.com/NiklasEi/bevy_game_template/blob/main/.github/workflows/release.yaml#L48
The current config will set this to the newest macOS version, which disables the app on all older machines.
I'm no expert at what a sensible default would be, but I'd wager a good candidate could be 11.0 Big Sur from November 19, 2020, since it is the first version to support universal binaries.

(credit for this discovery and suggested fix goes to @bash who's sitting next to me 😄)