Refactor error tracking gzip decompression

What does this MR do and why?

Error tracking has 2 endpoints: one of it uses middleware to decompress payload, another uses regular class. This MR makes both endpoints use the same middleware instead.

1. Add application/x-sentry-envelope support to lib/gitlab/middleware/compressed_json.rb middleware
2. Remove gzip decompression from lib/error_tracking/collector/sentry_request_parser.rb since we do it with middleware now.

We do this to remove code repetition.

No changes from user perspective.

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