Update the component part names

[oliverschuerch]

Regarding our discussion here, we must update some of our component part names and properly merge them to main.

User Story
As a developer of the Swiss Post Design System, I want to ensure our components internal ::part names are properly scoped to the component, so chances are smaller, that a selector can bleed through other components, creating unwanted side-effects.

Acceptance Criterias

• Update the part names of the post-accordion-item component.
  Already part of this pr fix(components, docs): fix post-accordion design discrepancies and remove or update post-accordion-item parts #6676.
• Update the part names of the post-menu component.
• Update the part names of the post-tabs component.
• Add an ADR out of the poll, which declares the naming convention (e.g. part names must align the component tag, followed by a dash and a single word explaining the element they represent).
• Implement a eslint rule to enforce the naming convention (there are some stencil related custom rules around already).

Be aware, all of these changes are breaking!