Remove ExpressionLanguage reference for 2.3 version

Remove "Using Expressions for Complex Security Restrictions" section using ExpressionLanguage in 2.3 version.

ExpressionLanguage is a 2.4 feature.
bug #4548 Remove ExpressionLanguage reference for 2.3 version (dangarzon)

This PR was merged into the 2.3 branch.

Discussion
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Remove ExpressionLanguage reference for 2.3 version

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3 only
| Fixed tickets |

Remove "Using Expressions for Complex Security Restrictions" section using ExpressionLanguage in 2.3 version.

ExpressionLanguage is a 2.4 feature.

Commits
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d1e7334 Remove ExpressionLanguage reference for 2.3 version
Merge branch '2.3' into 2.5

* 2.3:
  Remove ExpressionLanguage reference for 2.3 version
Fixing bad merge
Merge branch '2.3' into 2.5

* 2.3:
  Fixing bad merge
Reverts a commit to 2.3 for 2.4 features. This re-adds them to the 2.5 branch

Revert "Remove ExpressionLanguage reference for 2.3 version"

This reverts commit d1e7334bf94ac82c0f73d549d1b8543f0692c008.

Conflicts:
	best_practices/security.rst