Remove deprecated device context fields

[ueman]

There are a couple of fields which are deprecated in the device context. Those fields should be removed in v7.0. Follow up from #838

[philipphofmann]

@ueman, could you point out the deprecated fields in the description, please?

[ueman]

Here's the list:

device.language
device.timezone
device.screenResolution
device.theme

Speaking of removing deprecated fields, there are a couple more candidates:

• https://github.com/getsentry/sentry-dart/blob/6ae0ef03535461e6a75d1eb297e71376eaeed749/dart/lib/src/enricher/io_enricher_event_processor.dart#L95-L96 since it was moved to event.thread.name and can be enabled by options.attachThreads
• This runtime key https://github.com/getsentry/sentry-dart/blob/6ae0ef03535461e6a75d1eb297e71376eaeed749/dart/lib/src/enricher/io_enricher_event_processor.dart#L58 since it doesn't add much value as name is already indicating what it is
• Same for this key https://github.com/getsentry/sentry-dart/blob/6ae0ef03535461e6a75d1eb297e71376eaeed749/flutter/lib/src/flutter_enricher_event_processor.dart#L224 since it doesn't add much value as name is already indicating what it is

[ueman]

https://github.com/getsentry/sentry-dart/pull/934 introduced attaching http response data, so the MaxRequestBodySize and MaxResponseBodySize enums can be consolidated.