This PR was merged into the 4.1-dev branch.

Discussion
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Rename middlewares to middleware

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

`middleware` is uncountable in English (like software or information). So, I've removed the `s` everywhere. We can probably be a bit more explicit if we think it makes thing easier to understand like I've done in one class, like `middlewareHandler`.

Commits
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9ae7c11 Do not fix the `middleware` XML name anymore
52e7500 removed usage of middlewares

This PR was submitted for the master branch but it was squashed and merged into the 4.1 branch instead (closes #9727).

Discussion
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[Messenger] Update the messenger documentation

- [x] Fixes #9641 with the middleware configuration.
- [x] Fixes #9617 with the multiple bus configuration.
- [x] Change adapters to transports (waiting merge: symfony/symfony#27129)
- [x] middlewares config entry is renamed middleware (symfony/symfony#27177)
- [x] in the config, message buses names are the full service id you'll use (symfony/symfony#27162)
- [x] Add TransportInterface as first class citizen sender+receiver (symfony/symfony#27164)

Commits
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c3c3528 Few updates following review
64bfd75 Change wording and don't use `.`-based services so it's just clearer
e1f3b5a Fix the formating of the method name
9b7b85f Update the example of using multiple buses to use DI's `bind`s
c76b2c2 Uses the full service name when configuring the buses from the YAML configuration
2409798 Middleware does not have a plural
a20286d Add a note about the symfony serializer pack
10f46eb Introduce the `TransportInterface`
ef70bc0 Add a documentation about the middlewares
3ff8cfe Add multiple buses configuration and type-hint example
a4bc592 Rename the adapters to transport