chore(accordion): adds disabled property to CDK accordion-item (#8141)

[henetiriki]

Adds a disabled property to Accordion Item in the CDK.

Closes #8141

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[googlebot]

CLAs look good, thanks!

[jelbourn]

@ouq77 sorry if my comment on #8141 wasn't clear. I don't want to create a new subpackage in @angular/cdk for this type of behavior yet, as we don't have a well-defined set of features. What I meant instead was that we can just manually add the disabled property to the components.

[henetiriki]

I think I misunderstood @jelbourn - will revert and apply to the component directly.

Just to clarify: this means removing the disabled mixin from MatExpansionPanel and adding @Input() disabled to CdkAccordionItem, similar to expanded.

[josephperrott]

@ouq77 Correct

[henetiriki]

Changes applied. Waiting for your feedback. :)

[henetiriki]

@jelbourn and @josephperrott - yay for my 1st successful build! 😁

[henetiriki]

Hi @jelbourn - any update on this PR? I've noticed you've been quite busy getting v5 out, so no worries.

[josephperrott]

Sorry, not sure how I missed this one for so long.

LGTM

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