chore(billing): Bump sentry-protos to 0.18.0

[dashed]

Summary

• Bumps sentry-protos from 0.17.0 to 0.18.0
• 0.18.0: (billing) Add GrantSource, source field, and GetEffectiveGrants protos (sentry-protos#268)

All changes are additive — no breaking changes.

[github-actions]

Backend Test Failures

Failures on 63c03d2 in this run:

tests/symbolicator/test_minidump_full.py::SymbolicatorMinidumpIntegrationTest::test_full_minidump_json_extra — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/symbolicator/test_minidump_full.py:124: in test_full_minidump_json_extra
    event = self.post_and_retrieve_minidump(
src/sentry/testutils/relay.py:127: in post_and_retrieve_minidump
    assert resp.ok
E   assert False
E    +  where False = <Response [503]>.ok