Convert path alias migrations to create entities

Problem/Motivation

In #2336597: Convert path aliases to full featured entities, we convert path aliases to entities. Once that issue lands, we should update the migrations in the Path module to use entity:path_alias as the destination plugin instead of the custom url_alias.

Proposed resolution

Update migrations/d6_url_alias.yml and migrations/d7_url_alias.yml in the Path module. Change the destination key as noted and make any related changes that this requires.

Remaining tasks

• Update the migrations.
• Review automated tests.
• Create a change record.

User interface changes

None.

API changes

None.

Data model changes

None.