Build jdk21u HEAD weekly

[andrew-m-leonard]

Schedule jdk21u HEAD build at weekend, since no builds are being published upstream until January '24 CPU.

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[adamfarley]

LGTM

[sxa]

SGTM, but noting (as I have elsewhere) that this will override the default nightly/ea retrieved via the API so it is not an equivalent to a tag, however users are able to specify a specific tag (e.g. jdk-21+35-ea-beta or jdk-21.0.1+12-ea-beta ) as per the examples in the blog post about ea builds

[karianna]

the blog post about ea builds

I wonder if that blog post should be referenced from our nightly builds download pages