Use slot-based SchemaDefinitions when creating/singing block for Deneb

[StefanBratanov]

PR Description

We are specifically getting SchemaDefinitionsDeneb for milestone DENEB when creating/signing block

spec.forMilestone(SpecMilestone.DENEB).getSchemaDefinitions())

This breaks if we need to propose a block for later milestones where the block structure has changed. (e.g. Electra)

Fixed Issue(s)

N/A

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[tbenr]

LGTM