fix: SpanLimits setting event attributes length limit

[robertlaurin]

The spec allows for generic and more specific configuration of attribute length limits. https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute-limits

However we are currently applying legacy and span specific attribute environment variables to the event attribute length limit.

OTEL_EVENT_ATTRIBUTE_VALUE_LENGTH_LIMIT should take priority over OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT.

OTEL_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT and OTEL_RUBY_SPAN_ATTRIBUTE_VALUE_LENGTH_LIMIT should not configure the event attributes length limit.

Both span attribute length limit and event attribute length limit should use the more general OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT in the event a more specific configuration is not supplied.