[image_picker] Depend on AndroidX Core instead of Legacy Support v4

[ChristianKatzmann]

Description

It's not recommended to depend on the legacy-support-v4 artifact any longer as we should depend on the relevant sub-artifacts instead to reduce the transitive dependencies.

The image_picker package only uses classes from the androidx.core package, so we can safely replace the androidx.legacy:legacy-support-v4 dependency with androidx.core:core.

Related Issues

This simple change is part of the PR #1878.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• All existing and new tests are passing.
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• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

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[googlebot]

CLAs look good, thanks!

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[mklim]

LGTM. Thanks for the fix!