E.R: update ProGuard deobfuscation guide for Android R8

[melekr]

clones #3685, by @BartoszLitwiniuk.

Description

Updates the ProGuard deobfuscation page to reflect how Android builds actually work today: R8 replaced ProGuard in AGP 7.0 but emits the same mapping format, so the /proguard endpoint works unchanged and the page now says so instead of implying ProGuard is still the tool. Removes the two keep rules the page asked readers to add by hand, since the backtrace-android AAR has shipped its own consumer rules for a while now and -keep class com.google.gson.**.* no longer matches anything after Gson was shaded to backtraceio.gson. Added missed curl word in command to upload mappings.

Motivation and Context

Setting up Backtrace symbolication in a 2026 Android app on the latest backtrace-android surfaced several points where this page is outdated or actively misleading.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation fix (typos, incorrect content, missing content, etc.)

[github-actions]

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