[Form] Renamed form processors to request handlers
merged branch bschussek/improve-naming (PR #7732)

This PR was merged into the master branch.

Discussion
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[Form] Renamed form processors to request handlers

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/2092

Commits
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ae7c378 [Form] Renamed form processors to request handlers