Update dependency provider to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
provider	dependencies	major	5.0.0 -> 6.0.2

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Release Notes

rrousselGit/provider

v6.0.2

Added error details for provider that threw during the creation (thanks to @​jmewes)

v6.0.1

Removed the assert that prevented from using ChangeNotifierProvider
with notifiers that already had listeners.

v6.0.0

Compare Source

• Breaking: It is no longer possible to define providers where their
  only difference is the nullability of the exposed value:

  MultiProvider(
    providers: [
      Provider<Model>(...),
      Provider<Model?>(...)
    ]
  )

  Before, when defining those providers, doing context.watch<Model>
  vs context.watch<Model?> would point to a different provider.
  Now, both will always point to the deepest provider (in this example Provider<Model?>).

  That fixes issues where a common developer mistake is to define a provider
  as Model? but try to read it as Model. Previously, that would give a ProviderNotFoundException.
  Now, this will correctly resolve the value.

  That also helps large codebase migrate to null-safety progressively, as
  they would temporarily contain both null-safe and unsound code. Previously,
  unsound code would be unable to read a provider defined as Provider<Model?>,
  as Model? is not a valid syntax. With this change, this is now feasible.

• It is now possible to read a provider without throwing a ProviderNotFoundException
  if the provider is optional.

  To do so, when calling context.watch/context.read, make the generic type
  nullable. Such that instead of:

  context.watch<Model>()

  which will throw a ProviderNotFoundException if no matching providers are found,
  do:

  context.watch<Model?>()

  which will try to obtain a matching provider. But if none are found,
  null will be returned instead of throwing.

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