Fixed #35425 -- Avoided INSERT with force_update and explicit pk.

[jacobtylerwalls]

Trac ticket number

ticket-35425

Branch description

Allows force_update=True to issue an UPDATE when saving instances with explicit pk's (but where the field happened to have a default pk) as envisioned in ticket-29260.

Affected models where the primary key field is defined with a default or db_default, such as UUIDField.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

[sarahboyce]

Thank you!