Reuse WP_HTML_Processor::is_void() now that WP 6.5 is minimum supported version

[westonruter]

This removes a TODO to reuse WP_HTML_Processor::is_void() instead of replicating that functionality which was not available until WP 6.4. Now that 6.4 is the minimum-supported version, that method can be reused.

This could have been done previously in #1068 when the minimum version was bumped.

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[felixarntz]

Good catch @westonruter!

[felixarntz]

@westonruter Actually, it looks like https://github.com/WordPress/performance/actions/runs/8583685696/job/23573872147?pr=1115 has failures related to this change. Can you check?

[westonruter]

@felixarntz Ah, this may have been an oversight in core that certain void tags were missed. I've opened Core-60961 to discuss that. In the meantime, probably better to let this PR sit.

[felixarntz]

@westonruter SG. We should probably set the base to trunk then.

[westonruter]

Oh, and actually WP_HTML_Processor::is_void() was updated in 6.5 to include these deprecated void tags. So it's just that the version in 6.4 doesn't include them. So unless the change is backported to 6.4, this PR will have to wait until 6.5 is the minimum-supported version.