Update test fixture transitive dep to be php 8 compatible

```
[dependabot-core-dev] ~/dependabot-core/composer $ DEBUG_HELPERS=true rspec ./spec/dependabot/composer/update_checker_spec.rb:717
Run options: include {:locations=>{"./spec/dependabot/composer/update_checker_spec.rb"=>[717]}}

Randomized with seed 14137
php -d memory_limit=-1 /opt/composer/v2/bin/run
{"error":"Your requirements could not be resolved to an installable set of packages.\n  Problem 1\n    - doctrine\/instantiator 1.0.5 requires php >=5.3,<8.0-DEV -> your php version (8.2.1) does not satisfy that requirement.\n    - phpunit\/phpunit 5.1.3 requires phpunit\/phpunit-mock-objects >=3.0.5 -> satisfiable by phpunit\/phpunit-mock-objects[3.0.6].\n    - phpunit\/phpunit-mock-objects 3.0.6 requires doctrine\/instantiator ^1.0.2 -> satisfiable by doctrine\/instantiator[1.0.5].\n    - phpunit\/phpunit is locked to version 5.1.3 and an update of this package was not requested.\n"}
```

`doctrine/instantiator` is an indirect dep, so fine to bump w/o affecting
the unit test. So I ran `composer update doctrine/instantiator` to only
bump that dep (and any pins it required bumping) and then committing the results.

The test now passes.