Create official "Technical Requirements" for rules_android

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### Problem description (include actual vs expected text, if applicable):

Hi, in review for a change here, a Googler told me:

> We should really have a separate "Technical requirements" section in our README for this. We could start here: > [bazel_rules/rules_android/README.md]
>
> So far the technical requirements are:
> * Bazel 7.4+, 8.0+
> * Android SDK *

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### Any other information you'd like to share?

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Open bazel_rules/rules_android/README.md and review where its prerequisites are described. Add a separate Technical requirements section covering Bazel 7.4+, Bazel 8.0+, and the Android SDK; the work is done when those requirements are clearly documented in the README.

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