[HttpKernel] Removed deprecated Kernel::init() method
feature #12461 [HttpKernel] Removed deprecated Kernel::init() method (saro0h)

This PR was merged into the 3.0-dev branch.

Discussion
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[HttpKernel] Removed deprecated Kernel::init() method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | maybe
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Commits
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37b49e1 [HttpKernel] Removed deprecated Kernel::init() method