firestore_exceptions.h: remove dependency on absl to set FIRESTORE_HAVE_EXCEPTIONS

[dconeybe]

firebase-cpp-sdk has some issues compiling absl-dependent code on Android; therefore, remove the dependency on absl from firestore_exceptions.h, as it was before #14348.

See https://github.com/firebase/firebase-ios-sdk/pull/14348/files/1989942cffe222e986230c1539908a9fd105360c#r1950085884

#no-changelog

[dconeybe]

firebase-cpp-sdk test run to demonstrate that this PR fixes the android build breakage: https://github.com/firebase/firebase-cpp-sdk/actions/runs/13254450771/job/36998710412?pr=1689

[jonsimantov]

Looks good. Can you keep this branch open until the C++ release is out in the next day or two? I temporarily changed the Firestore external reference to point directly to it and I don't want it to go away before the package is finished building.

[dconeybe]

Looks good. Can you keep this branch open until the C++ release is out in the next day or two? I temporarily changed the Firestore external reference to point directly to it and I don't want it to go away before the package is finished building.

I think the dconeybe/AndroidAbslExceptionsFix branch will be automatically deleted when this PR is merged. I can definitely delay the merge until you are done with it, since the cpp sdk is the only code (that I am aware of) that cares about this PR. Or you can point your code at the commit hash in the main branch after this PR is merged. Your call.

[jonsimantov]

Hmm, let's use the commit hash then. I'll make the change on the C++ side.

[dconeybe]

Hmm, let's use the commit hash then. I'll make the change on the C++ side.

Ok. PR has been merged. The commit hash is 25e724d