Added tbachert/spi to config.allow-plugins of composer.json.

[AlexSkrypnyk]

Drupal core 10.3.6 aded dependency on tbachert/spi and it is now needs to be added to config.allow-plugins of composer.json.

$ composer why -r tbachert/spi
drupal-composer/drupal-project 1.x-dev requires (for development) drupal/core-dev (^10.2.0)         
drupal/core-dev                10.3.6  requires                   open-telemetry/exporter-otlp (^1) 
drupal/core-dev                10.3.6  requires                   open-telemetry/sdk (^1)           
open-telemetry/api             1.1.0   conflicts                  open-telemetry/sdk (<=1.0.8)      
open-telemetry/exporter-otlp   1.1.0   requires                   open-telemetry/sdk (^1.0)         
open-telemetry/sdk             1.1.0   requires                   tbachert/spi (^1.0.1)       

[AlexSkrypnyk]

@leymannx
sorry, I had to merge this to unblock builds.