(backport) Fix race conditions leading to database errors #15386

Job	Run time
Build and check documentation	1m 47s
Run the Django test suite in reverse	5m 48s
Set up docker build 'dynamic' env variables	1s
End-to-end tests, chromium	15m 51s
Run the Django test suite	8m 10s
End-to-end tests, firefox	18m 3s
End-to-end tests, webkit	14m 26s
docker_build (production) / Build image	2m 53s
docker_build (extensions, token_exchange,prefill_haalcentraalhr, all-extensions-) / Build image	2m 47s
Scan docker image	2m 34s
Simulate upgrading instances (2.8.2)	1m 19s
Simulate upgrading instances (2.8.3)	1m 21s
Check for unexepected OAS changes	7s
Generate Postman collection from OAS	10s
Validate OAS	16s
Generate SDKs from OAS	41s
Push Docker image	0s
update-docker-readme	0s
	1h 16m 14s

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