composefs: Use `uki` key for Type 1 configs pointing to a UKI

[travier]

We should now use the "new" uki key from the BLS spec in Type 1 configs when referencing a UKI. See: https://uapi-group.org/specifications/specs/boot_loader_specification/#type-1-boot-loader-entry-keys

To not break updates, we should still accept configs using the efi key but newly generated configs should use the uki key only.

Both systemd-boot and the grub-cc build should support that now.

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From https://discussion.fedoraproject.org/t/f45-change-proposal-grub-efi-for-confidential-computing-self-contained/193574/7

[cgwalters]

Both systemd-boot and the grub-cc build should support that now.

But I think not the version in c10s - we may need to do some kind of runtime detection

[Johan-Liebert1]

Ref systemd/systemd#39886

"uki" stanza in bls type 1 entries was added in v258

we'll need runtime detection for systemd version