[FrameworkBundle] Update deprecated service call

The service `security.context` is now deprecated due to PR #11690.

This commit updates PR #11593 replacing `security.context` with
`security.authorization_checker` in the `isGranted()` controller
shortcut.
minor #12787 [FrameworkBundle] Update deprecated service call (lrlopez)

This PR was merged into the 2.6 branch.

Discussion
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[FrameworkBundle] Update deprecated service call

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none, but related to #11593 and #11690
| License       | MIT
| Doc PR        | none

Service `security.context` is now deprecated due to PR #11690. This commit updates PR #11593 replacing `security.context` with `security.authorization_checker` in the `isGranted()` controller shortcut introduced in Symfony 2.6.

Commits
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2625193 [FrameworkBundle] Update deprecated service call