Alter cache tags after HtmlResponseSubscriber

Problem/Motivation

Cache tags for placeholders are bubbled by HtmlResponseSubscriber::onRespond() so cache_tags_simplify should simplify the tags after that.
However, it still needs to come before CacheableResponseSubscriber::onRespond(), so we first need #3274608: Use a lower priority for CacheableResponseSubscriber::onRespond() → then we would be able to use a priority between these two event subscribers.

Proposed resolution

Use a priority between 0 and the new priority set in #3274608: Use a lower priority for CacheableResponseSubscriber::onRespond() → .