Update cakephp/cakephp-codesniffer requirement from ^4.5 to ^5.1

Created by: dependabot[bot]

Updates the requirements on cakephp/cakephp-codesniffer to permit the latest version.

Release notes

Sourced from cakephp/cakephp-codesniffer's releases.

5.1.0

What's Changed

• Add sniff for unused variables. by @​ADmad in cakephp/cakephp-codesniffer#371

Full Changelog: https://github.com/cakephp/cakephp-codesniffer/compare/5.0.1...5.1.0

Commits

• 0543c54 Merge pull request #371 from ADmad/unused-variables
• 9608a82 Add sniff for unused variables.
• 6cbe404 Merge pull request #366 from cakephp/5.x-fn-spacing
• 90baca0 Add spacing check for arrow function closures.
• 947b2ac Merge pull request #365 from cakephp/ADmad-patch-1
• 16b7673 Bump up slevomat/coding-standard
• bb4f82f Merge pull request #364 from cakephp/intersection-sniff
• 21654b9 Add test for intersections to type hint sniff
• c368beb Rename legacy generics to phpstorm generics to match psalm
• e281a50 Merge pull request #362 from cakephp/5.x-update-slevomat-standard
• Additional commits viewable in compare view

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