chore(discover): Update max fields

[wmak]

• Increases max fields to 50 since we're starting to bump up against the limit

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
26620	1	26619	231

View the top 1 failed test(s) by shortest run time

tests.snuba.api.endpoints.test_organization_events.OrganizationEventsEndpointTest::test_limit_number_of_fields

Stack Traces | 35.8s run time

#x1B[1m#x1B[.../api/endpoints/test_organization_events.py#x1B[0m:4161: in test_limit_number_of_fields
    assert response.status_code == 400
#x1B[1m#x1B[31mE   assert 200 == 400#x1B[0m
#x1B[1m#x1B[31mE    +  where 200 = <Response status_code=200, "application/json">.status_code#x1B[0m

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