🔖 Bump version to 2.7.8 and update release notes #13910

Job	Run time
Run the Django test suite	7m 7s
Build and check documentation	3m 9s
Set up docker build 'dynamic' env variables	1s
End-to-end tests, chromium	11m 49s
Run the Django test suite in reverse	5m 59s
End-to-end tests, firefox	15m 32s
End-to-end tests, webkit	12m 21s
docker_build (utrecht_zds, stuf_zds_payments, utrecht-zds-) / Build image	2m 39s
docker_build (production) / Build image	2m 39s
docker_build (extensions, token_exchange,prefill_haalcentraalhr, all-extensions-) / Build image	2m 38s
Check for unexepected OAS changes	4s
Simulate upgrading instances (2.6.7)	1m 15s
Scan docker image	2m 11s
Simulate upgrading instances (2.6.13)	1m 20s
Generate SDKs from OAS	3m 18s
Validate OAS	16s
Generate Postman collection from OAS	12s
Push Docker image	0s
update-docker-readme	0s
	1h 12m 30s

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