[camera_android_camerax] Bump cameraxVersion to 1.6.2, remove concurrent-futures workaround

[dhc-tech]

Bumps cameraxVersion from 1.6.1 to 1.6.2. This release includes an upstream
fix (https://issuetracker.google.com/issues/543937345) removing the JSpecify
type-use annotation that required consumers to explicitly declare
androidx.concurrent:concurrent-futures on their compile classpath. With
1.6.2, that workaround (added in #12373) is no longer needed, so this PR
removes the explicit dependency and its regression test.

Verified the fix is actually present in the published 1.6.2 artifact by
decompiling SurfaceRequest.class from camera-core-1.6.2.aar — the
mSurfaceRecreationCompleter field no longer carries the JSpecify
@NonNull type annotation that caused the original compile failure.

Fixes flutter/flutter#190505

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets.
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style (bumped to 0.7.4+8).
• I updated/added any relevant documentation (doc comments with ///). N/A — no public API surface changed, only a build-file dependency version bump.
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under. This PR removes a dependency and its guard test rather than adding new behavior — the removed test (ConcurrentFuturesDependencyTest) existed solely to guard the now-unnecessary workaround dependency from [camera_android_camerax] Add explicit concurrent-futures dependency #12373, so deleting it alongside the dependency is correct rather than requiring a replacement test.
• All existing and new tests are passing.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates the cameraxVersion from 1.6.1 to 1.6.2 in build.gradle.kts and removes the explicit androidx.concurrent:concurrent-futures dependency workaround along with its associated regression test, ConcurrentFuturesDependencyTest.java. The package version is bumped to 0.7.4+8 in pubspec.yaml and documented in CHANGELOG.md. There are no review comments, and I have no feedback to provide.