feat(base,app)!: GitHub signs nobody in any more (ADR 0020, D36)

Every account signs in with SimpleLogin or a password now, so GitHub
goes from the module, the shared app package, Learn and Todo, ahead of
ADR 0017's GITHUB_SIGN_IN_ENDS.

- halkyon_base: no `gitHubSignIn` endpoint, GitHub identity provider or
  GitHub refusals; no `githubClientId`/`githubClientSecret`, no
  `GITHUB_SIGN_IN_ENDS` and no `forgetGitHubAccounts`; the register
  loses `account-with-github`. `SignInWays.gitHub`, the basics'
  `gitHubClientId` and `gitHubSignInEnds`, and `PasswordRefusal`'s
  `signInAgain` and `noAddress` go. A first password takes a code
  mailed to the person's own address; an account without one sets none
  (`wrongCredentials`).
- Learn's server: `instance.info()` without `gitHubClientId`, the
  pruning without GitHub, and `minimumAppBuild` 24, as builds up to 23
  (v0.20.0) require `SignInWays.gitHub`: the server deploys with the
  release of build 24.
- halkyon_app: no GitHub button or problems on the start screen, no
  password offered after a GitHub sign-in, and `ServerpodAuthGateway`
  without the browser half only GitHub used (`(client, base:)`). The
  account section offers a first password only to an account with an
  address. `LoopbackWebAuth` and the apps' schemes stay, SimpleLogin's
  ways back.
- Learn's Profile lists no GitHub; the texts go (English, German).
- Serverpod's `serverpod_auth_idp_github_account` stays, as its
  migrations make it; a row goes with its account, and ADR 0020's D36
  gives operators the SQL to delete those left.
- deploy/: compose.yaml and .env.example without GitHub. The READMEs,
  AGENTS.md files and ADR 0017 amended.

BREAKING CHANGE: builds up to 23 can't read the instance's answers
about an account's ways in; minimumAppBuild is 24.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/unit/password_config_test.dart

@@ -61,34 +61,6 @@ void main() {
       );
     });
   });
-
-  group('The end of GitHub sign-in', () {
-    test('is the start, in UTC, of the day the operator names', () {
-      expect(gitHubSignInEndsFrom('2027-01-22'), DateTime.utc(2027, 1, 22));
-      expect(gitHubSignInEndsFrom(' 2027-01-22 '), DateTime.utc(2027, 1, 22));
-    });
-
-    test('is none while unset or empty', () {
-      expect(gitHubSignInEndsFrom(null), isNull);
-      expect(gitHubSignInEndsFrom(''), isNull);
-    });
-
-    test('is refused for anything but a day, so the server stops', () {
-      for (final value in [
-        '2027-1-22',
-        '22.01.2027',
-        '2027-02-30',
-        '2027-01-22T00:00:00Z',
-        'soon',
-      ]) {
-        expect(
-          () => gitHubSignInEndsFrom(value),
-          throwsStateError,
-          reason: value,
-        );
-      }
-    });
-  });
 }
 
 extension on Map<String, String> {