[CT-2910] [Mesh] Restructure ContractBreakingChangeError message to explicitly list breaking changes

[MichelleArk]

Housekeeping

• I am a maintainer of dbt-core

Short description

The current error message:

Breaking Change to Contract Error in model model_x (model_x.sql)
  While comparing to previous project state, dbt detected a breaking change to an enforced contract.
  
  The contract's enforcement has been disabled.

results in confusion because it's unclear that the "The contract's enforcement has been disabled." is the breaking change, as opposed to an effect that dbt had as a result of a command.

The ContractBreakingChangeError already aggregates breaking changes into a list, so it should be a matter of updating the formatting here!

Acceptance critera

The error message is formatted as a list, where each list element is a breaking change (more than one are possible in the scope of a single ContractBreakingChangeError exception)

For example,

While comparing to previous project state, dbt detected one or more breaking changes to a model with an enforced contract.
  - Contract enforcement was removed: Previously, this model's configuration included contract: {enforced: true}. It is no longer configured to enforce its contract, and this is a breaking change.
  - Columns were removed:
     - this_column_will_be_removed
     - this_one_too
  - Columns with data_type changes:
     - id (integer -> text)
     - another_column (date -> string)

Context

Refinement issue: #8029 (includes reproduction steps)