No deprecation warning for `cdk-focus-initial` on console or change log

[emilio-martinez]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Use of 'cdk-focus-initial' will result in printing the following warning

Found use of deprecated attribute 'cdk-focus-initial', use 'cdkFocusInitial' instead.

What is the current behavior?

No message is printed because this._element is checked for a cdk-focus-initial attribute rather than redirectToElement. See reference to relevant lines below—shown is v5.2.4, but the same applies to master.

https://github.com/angular/material2/blob/5.2.4/src/cdk/a11y/focus-trap/focus-trap.ts#L169-L185

What are the steps to reproduce?

The below repro is basically the same as the example on the Angular Material website, but cdk-focus-initial instead of cdkFocusInitial.

https://stackblitz.com/edit/angular-material2-deprecated-cdk-focus-initial

What is the use-case or motivation for changing an existing behavior?

Correctly showing deprecation warning.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Versions of @angular/cdk since c663fad was introduced in 5.0.0-rc0.

Is there anything else we should know?

The changelog for 5.0.0-rc0 only noted the attribute change to camel case (quote shown below) but did not note it as a deprecation, which is problematic because users haven't been getting this warning at all so far as shown above.

focus-trap: update focus trap attrs to camel case #6799 (#6960) (c663fad)

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