require LTSS channels for SLE 15 SP1, SLE 15 SP2 and SLE 15 SP3

[mcalmer]

What does this PR change?

Fix bootstrap data and require LTSS channels for SLE15 SP1 up to SP3 to get the new dependency package.

Links

Port of https://github.com/SUSE/spacewalk/pull/22031

• DONE

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