[Symfony 6] Bring stackphp/builder into core for Symfony 6 compatibility and remove it as a dependency

Yeah, looks like Symfony's approach is just to declare all this at service configuration time. The PR that added this feature has more examples: https://github.com/symfony/symfony/pull/36373

If we don't want to do this just yet we could copy StackedHttpKernel into core and update it for Symfony 6, it's only one class.

I've read the PR in detail and I think it is missing some parts for us to be able to use it.

We order services by priority and modules can decide to insert services at any level into the middleware stack, e.g. the page_cache middleware inserts itself into the middle of the chain. As far as I can see Symfony hasn't yet implemented this, as per the PR:

Using configuration, we could also imagine new tags. E.g. one to allow filtering a frame while embedding a stack, another to allow replacing a frame by a new one, another for inserting a frame before/after another one - doing so either while embedding and/or by altering an existing stack definition. I didn't mention it, but frames can be named, this could help.

While these are suggested as future features in the PR, there have been no relevant changes to the compiler pass to implement this: https://github.com/symfony/dependency-injection/commits/6.1/Compiler/Res...

The page cache middleware also has a Drupalism in the responder property, which makes all subsequent services lazy, in the assumption that the responder will not usually be calling further down the stack. This isn't implemented in the Symfony stack syntax, although we could probably do it as a subsequent compiler pass.

However, as far as I can see we need the ability to modify existing stacks and Symfony not providing that yet is a deal breaker for us. Therefore I suggest we copy StackedHttpKernel into core and continue to use it as a middleware provider as the easiest route forward.

This patch copies Stack\StackedHttpKernel to Drupal\Core\StackMiddleware\StackedHttpKernel, updates all references and tests, and removes stack/builder as a dependency.

Fix coding standards.

Fixing failing tests

All the code changes look good to me.
The class from the dependency that we use is copied to Drupal. Therefor we do not need the package anymore.
Adding the tag for a release manager review as we are removing a dependency.
For me it is RTBC.

Updated issue title to match the chosen approach. Issue summary needs updating to match, will try to get to it tomorrow if nobody else gets there first.

Update the IS.

Committed c97b854 and pushed to 10.0.x. Thanks!

I re-did the composer.lock to resolve conflicts.

Tagging for release notes mention.

The release note here says that there is no functional change, but presumably at least use statements need to be updated to load the thing from the core namespace instead? Should it have a small change record for that?

Confirmed that there are no contrib usages, thanks. Updating the release note to clarify that the service is unchanged.