Use user.FullName in Oauth2 id_token response (#6071)

Cherry-pick of [gitea#32542](https://github.com/go-gitea/gitea/pull/32542).

This makes /login/oauth/authorize behave the same way as the /login/oauth/userinfo endpoint. Previously, `name` property of the returned OIDCToken used to depend on the UI.DefaultShowFullName setting (I don't think this is desired behavior). Even worse, the `userinfo` endpoint can return basically the same data, but the `name` value there always returned `FullName`, even if it's empty (no fallback to `Name`).

A few notes:

I'm not sure what branch to target with this PR, please correct me if I'm chose the wrong one.

The deleted lines in the tests are duplicates, there's a copy of the whole thing just below, the only difference being the `Name` field (used to test the dependency on the UI.DefaultShowFullName setting)

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6071
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Baltazár Radics <baltazar.radics@gmail.com>
Co-committed-by: Baltazár Radics <baltazar.radics@gmail.com>

routers/web/auth/oauth.go

@@ -231,7 +231,7 @@ func newAccessTokenResponse(ctx go_context.Context, grant *auth.OAuth2Grant, ser
 			Nonce: grant.Nonce,
 		}
 		if grant.ScopeContains("profile") {
-			idToken.Name = user.GetDisplayName()
+			idToken.Name = user.DisplayName()
 			idToken.PreferredUsername = user.Name
 			idToken.Profile = user.HTMLURL()
 			idToken.Picture = user.AvatarLink(ctx)
@@ -305,7 +305,7 @@ func InfoOAuth(ctx *context.Context) {
 
 	response := &userInfoResponse{
 		Sub:      fmt.Sprint(ctx.Doer.ID),
-		Name:     ctx.Doer.FullName,
+		Name:     ctx.Doer.DisplayName(),
 		Username: ctx.Doer.Name,
 		Email:    ctx.Doer.Email,
 		Picture:  ctx.Doer.AvatarLink(ctx),