Convert array usage to `[]` instead

[keevan]

100% agree. But, formatting should be fixed in a different PR.

Originally posted by @dmitriim in https://github.com/catalyst/moodle-tool_objectfs/pull/473#discussion_r833833788

This can be done quickly using something like phpcbf

phpcbf path/to/code --standard=Generic --sniffs=Generic.Arrays.DisallowLongArraySyntax