Align BankID app autostart with current BankID guidelines

[torselden]

Updates how Active Login launches the BankID app so it follows BankID's current autostart guidance. Autostart is the default on all supported devices, the recommended universal/app link is used on mobile, and the manual launch button is only shown as a fallback where a browser actually requires user interaction.

This PR fixes #534

High level overview of this PR

• Use the universal/app link (https://app.bankid.com/) for all iOS and Android browsers, and keep the bankid:/// scheme for desktop
• Launch the app via BankID's recommended anchor-click with referrerPolicy="origin" instead of window.location.href (fixes unreliable iOS universal-link launch)
• Restrict the manual "start BankID" fallback button to Android browsers that require a user gesture (Chrome, Edge, Samsung Internet, Brave); keep autostart automatic on iOS and on Android Firefox/Opera

These things have been implemented (when relevant)

• Code written
• Test added — launcher tests for app-link vs scheme selection, the user-interaction matrix, and redirect parameter behavior (Android redirect=null, iOS Safari return URL, iOS Chrome/Firefox browser scheme, iOS Edge/Opera redirect=null)
• Documentation updated / written — added a "How the BankID app is launched" section and a mobile browser-support note in bankid.md

Backwards-compatibility note
This PR changes the launch URL selection for several device/browser combinations (no public API changes):

• CanUseAppLink no longer restricts the app link to Safari-on-iOS and Chrome/Edge-on-Android ≥ 6 — it now applies to every iOS and Android browser. The DeviceOsVersion.MajorVersion >= 6 guard was removed.
  As a result, iOS non-Safari (Chrome, Firefox, Edge, Opera) and Android Firefox/Opera / older Android now receive https://app.bankid.com/… where they previously received bankid:///…. This matches current BankID guidance and should improve reliability.

• The manual "start BankID" fallback button is now scoped to restricted Android browsers instead of a broader set.
  What consumers should verify: if you relied on the old scheme-based launch for a specific browser, that path changes. Android Firefox/Opera in particular should be verified on real devices, since their historical app-link/intent handling has been inconsistent. Custom IBankIdLauncher / IBankIdSupportedDeviceDetector implementations are unaffected, and the deprecated redirect query parameter is still emitted, so BankID app compatibility is preserved.

[elinohlsson]

Looks really good! I will just do some manual testing before merging.

[elinohlsson]

I made a couple of changes based on testing the BankID autostart flow across our supported mobile browsers:

• Keep BankID's recommended App Link approach (https://app.bankid.com/) as the default for mobile.
• On Android, Firefox and Samsung Internet are explicit exceptions. Our testing shows that the App Link does not launch the BankID app in these browsers, so they fall back to the bankid:/// scheme.
• I tested reverting the new JavaScript launch implementation to window.location.href, but that did not change the behavior. This suggests the issue is specific to App Link support in these browsers rather than launchBankIdApp().
• Updated the browser support documentation to describe the Android exceptions.
• Removed the outdated IE11/polyfill documentation.
• Removed Opera from the verified browser list for now, since the redirect flow is currently not working there.

With these changes, the BankID launch flow works for all of the Android browsers we currently test before release, as well as all of the iOS browsers we test.

There are still some existing redirect issues that need to be addressed, particularly with Edge, and Opera support needs to be investigated. These issues were present before this PR and are not introduced by these changes, so I suggest we look into them separately in another PR.