Rename SectionStorageTrait to SectionListTrait

Problem/Motivation

\Drupal\layout_builder\SectionListInterface provides methods for manipulating a list of sections.
\Drupal\layout_builder\SectionStorageInterface extends that interface to provide methods to load and persist that list.

\Drupal\layout_builder\Plugin\SectionStorage\SectionStorageBase exists to provide a base class for implementations of SectionStorageInterface

\Drupal\layout_builder\SectionStorage\SectionStorageTrait exists to provide a trait for implementations of SectionListInterface

That name mismatch is rather confusing.

Proposed resolution

Move the code to a new trait named SectionListTrait and empty out and deprecate the old trait, importing/using the new one for BC.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet