fix: Update related objects with unique_together

[zvyn]

Description

Resolves the issue described in #360: Support updating related objects with unique_together on the foreign key where the data identifies a single object in the database.

Types of Changes

• [x] Bugfix
• [x] Enhancement/optimization

Issues Fixed or Closed by This PR

• 360

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov-commenter]

Codecov Report

Patch coverage: 66.66% and no project coverage change.

Comparison is base (4cde249) 87.99% compared to head (9f01759) 87.99%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #362 +/- ## ======================================= Coverage 87.99% 87.99% ======================================= Files 33 33 Lines 2940 2941 +1 ======================================= + Hits 2587 2588 +1 Misses 353 353 ``` | [Files Changed](https://app.codecov.io/gh/strawberry-graphql/strawberry-graphql-django/pull/362?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=strawberry-graphql) | Coverage Δ | | |---|---|---| | [strawberry\_django/mutations/resolvers.py](https://app.codecov.io/gh/strawberry-graphql/strawberry-graphql-django/pull/362?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=strawberry-graphql#diff-c3RyYXdiZXJyeV9kamFuZ28vbXV0YXRpb25zL3Jlc29sdmVycy5weQ==) | `87.05% <66.66%> (+0.05%)` | :arrow_up: |

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