Remove legacy API endpoints and compatibility code dropped upstream (#7809)

Remove endpoints, response fields, feature flags and compatibility code
that upstream Bitwarden has removed and that no current client uses:

- Legacy POST /identity/accounts/register and POST /api/accounts/prelogin;
  register/finish is rate limited like the other unauthenticated endpoints.
- Legacy organization routes: PUT groups/{id}/users, PUT policies/{type}/vnext,
  PUT users/{id}/reset-password, GET billing/metadata, and the POST aliases of
  the organization, collection, group and member PUT/DELETE routes.
- POST /two-factor/disable (PUT stays).
- ResetMasterPassword, captchaBypassToken, the legacy registration name field,
  userIsManagedByOrganization/managedByOrganization and accessAll.
- The automatic email 2FA code for clients older than 2025.5.0.
- DUO_USE_IFRAME and the Duo Traditional Prompt code.
- Experimental client feature flags that no client reads anymore.
- Web vault CSS selectors for web vaults older than 2025.5.1.

Also accept the PascalCase fields Android 2026.9.0+ sends on registration
and password change, and align a few account, login/2FA and organization
checks with upstream.

Adds tests for the registration body formats.

src/static/templates/scss/vaultwarden.scss.hbs

@@ -42,45 +42,27 @@ a[href$="/settings/sponsored-families"] {
 {{/if}}
 
 /* Hide the `Enterprise Single Sign-On` button on the login page */
-{{#if (webver ">=2025.5.1")}}
 {{#if (not sso_enabled)}}
 .vw-sso-login {
   @extend %vw-hide;
 }
 {{/if}}
-{{else}}
-app-root ng-component > form > div:nth-child(1) > div > button[buttontype="secondary"].\!tw-text-primary-600:nth-child(4) {
-  @extend %vw-hide;
-}
-{{/if}}
 
 /* Hide the `Log in with passkey` settings */
 app-user-layout app-password-settings app-webauthn-login-settings {
   @extend %vw-hide;
 }
 /* Hide Log in with passkey on the login page */
-{{#if (webver ">=2025.5.1")}}
 .vw-passkey-login {
   @extend %vw-hide;
 }
-{{else}}
-app-root ng-component > form > div:nth-child(1) > div > button[buttontype="secondary"].\!tw-text-primary-600:nth-child(3) {
-  @extend %vw-hide;
-}
-{{/if}}
 
 /* Hide the or text followed by the two buttons hidden above */
-{{#if (webver ">=2025.5.1")}}
 {{#if (or (not sso_enabled) sso_only)}}
 .vw-or-text {
   @extend %vw-hide;
 }
 {{/if}}
-{{else}}
-app-root ng-component > form > div:nth-child(1) > div:nth-child(3) > div:nth-child(2) {
-  @extend %vw-hide;
-}
-{{/if}}
 
 /* Hide the `Other` button on the login page */
 {{#if (or (not sso_enabled) sso_only)}}
@@ -154,17 +136,10 @@ ng-dropdown-panel div.ng-dropdown-panel-items div:has(> [title="DuckDuckGo"]) {
   take a while for the change to take effect. To avoid the button appearing
   when it shouldn't, we'll keep this style in place for a couple of versions */
 /* Hide the register link on the login screen */
-{{#if (webver "<2025.3.0")}}
-app-login form div + div + div + div + hr,
-app-login form div + div + div + div + hr + p {
-  @extend %vw-hide;
-}
-{{else}}
 app-root a[routerlink="/signup"] {
   @extend %vw-hide;
 }
 {{/if}}
-{{/if}}
 
 {{#if remember_2fa_disabled}}
 /* Hide checkbox to remember 2FA token for 30 days */