Replace Orfox references with Tor Browser for Android

[heathdutton]

Fixes #6318

Description

Orfox was discontinued in 2019 and replaced by Tor Browser for Android. This updates the mobile browser warning and related code:

• Update warning text to reference "Tor Browser for Android" instead of "Orfox"
• Fix the TB4A_UA_REGEX to match modern Tor Browser for Android user agents (which use 3-digit Gecko versions like Gecko/115.0)
• Update test fixture and test names to reflect the change

Test plan

• Verify warning displays correctly when accessing Source Interface with Tor Browser for Android
• Existing functional test test_warning_appears_if_tor_browser_android_is_used covers this behavior

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy