Fix build

[staabm]

by a recent release of dotrince/instantiator 2.1.x it seems we now need to explicitly declare doctrine/instantiator:^1.0 for the PHPUnit downgrade.

I don't know exactly why that is, but it seems with this PR we are at least back to the state we had yesterday and no longer have additional errors we just saw in #4724 like

Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - doctrine/instantiator is locked to version 2.1.0 and an update of this package was not requested.
    - doctrine/instantiator 2.1.0 requires php ^8.4 -> your php version (8.2.99; overridden via config.platform, actual: 7.4.33) does not satisfy that requirement.
  Problem 2
    - phpunit/phpunit is locked to version 9.6.31 and an update of this package was not requested.
    - doctrine/instantiator 2.1.0 requires php ^8.4 -> your php version (8.2.99; overridden via config.platform, actual: 7.4.33) does not satisfy that requirement.
    - phpunit/phpunit 9.6.31 requires doctrine/instantiator ^1.5.0 || ^2 -> satisfiable by doctrine/instantiator[2.1.0].

Error: Your lock file does not contain a compatible set of packages. Please run composer update.

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

Thank you!