dropdown_language block redirects to wrong URL when switching language on Drupal 11.3.x

Problem/Motivation

On Drupal 11.3.10, switching language via the dropdown_language block redirects to the first visited page(after cache clear) instead of the current page in the selected language.

• #2232375: Support CacheOptionalInterface in BlockViewBuilder, use it in language switcher block to prevent max-age 0 bubbling
• Change record #3554070: Block plugins implementing CacheOptionalInterface will not have their own render cache entries

Proposed resolution

The fix is to implement CacheOptionalInterface on the DropdownLanguage block class, identical to how Drupal core's own LanguageBlock was updated in https://www.drupal.org/project/drupal/issues/2232375, so that cache keys are never assigned and the block always renders fresh with the correct current page URL.

Drupal core's own LanguageBlock:

https://git.drupalcode.org/project/drupal/-/blob/11.3.11/core/modules/la...