Fix betterproto2 migration: update generated proto code and callers (#765)

The dependency switch from betterproto to betterproto2 was incomplete.
This updates all affected files to use the betterproto2 API:

- tombstone.py: rewrite generated code to use betterproto2.field() with
  explicit TYPE_* constants, repeated/optional/group flags, and map_meta()
  for map fields
- tombstone_crashes.py: update import and fix to_dict() call to use
  keyword-only casing= argument required by betterproto2
- pyproject.toml: replace betterproto[compiler] dev dep with betterproto2-compiler
- Makefile: update protoc plugin flag to --python_betterproto2_out

pyproject.toml

@@ -57,7 +57,7 @@ dev = [
     "stix2>=3.0.1",
     "ruff>=0.1.6",
     "mypy>=1.7.1",
-    "betterproto[compiler]",
+    "betterproto2-compiler",
 ]
 
 [build-system]