dosbox-staging: downgrade to 0.78.0
dosbox-staging: downgrade to 0.78.0

`dosbox-staging` was updated to 0.78.1 because upstream had published
a `v0.78.1` tag (since removed). This used the stable tag format but
there was no corresponding release on GitHub, no announcement on the
homepage, etc.

Upstream confirmed this wasn't a release version and `v0.78.1-rc`
would have been a more appropriate tag. It's unclear when they will
release a new version, so this commit downgrades the formula to the
latest stable version, `0.78.0`.

This also adds a `livecheck` block using the `GithubLatest` strategy,
to align with the upstream release process (i.e., a version tag isn't
a sufficient indicator of a new release and we need to check GitHub
releases instead).

Closes #92523.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

Homebrew/homebrew-core,nandub/homebrew-core,zyedidia/homebrew-core,uyjulian/homebrew-core,nbari/homebrew-core,sjackman/homebrew-core,makigumo/homebrew-core,uyjulian/homebrew-core,chrisfinazzo/homebrew-core,lemzwerg/homebrew-core,filcab/homebrew-core,nandub/homebrew-core,stgraber/homebrew-core,mfikes/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,stgraber/homebrew-core,adam-moss/homebrew-core,sjackman/homebrew-core,nbari/homebrew-core,chrisfinazzo/homebrew-core,adam-moss/homebrew-core,cblecker/homebrew-core,makigumo/homebrew-core,cblecker/homebrew-core,zyedidia/homebrew-core,filcab/homebrew-core,mfikes/homebrew-core