Downgrade collection to 1.17.2

[deepak786]

Note: collection is pinned to version 1.17.2 by flutter_test from the flutter SDK.
See https://dart.dev/go/sdk-version-pinning for details.


Because intercom_flutter depends on flutter_test from sdk which depends on
  collection 1.17.2, collection 1.17.2 is required.
So, because intercom_flutter depends on collection ^1.18.0, version solving
  failed.

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