[camera_android_camerax] Update Android minimum SDK version

[camsim99]

#9853 bumped it to 23, so updating the README.

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[gemini-code-assist]

Code Review

This pull request updates the minimum Android SDK version in the camera package's README from 21 to 23. While the change in the support table is correct, I've pointed out that the Android setup instructions in the same file still refer to SDK 21. I've recommended updating this section as well to ensure the documentation is consistent and to prevent confusion for developers.

[gemini-code-assist]

While this update to SDK 23 in the support table is correct, the Android setup instructions further down in this file (lines 40-44) still refer to minSdkVersion 21. To maintain consistency and avoid user confusion, please update that section to minSdkVersion 23 as well.

[camsim99]

Overriding version/changelog since this is a docs only change.