Bump actions/cache to v4 #1618

Job	Run time
Test Demo Relying Party (18.x)	17s
Test Package (Python 3.10, Django 4.2)	2m 33s
Test Demo Relying Party (20.x)	17s
Test Package (Python 3.10, Django 5.0)	2m 55s
Test Package (Python 3.10, Django 5.1)	3m 37s
Test Package (Python 3.10, Django main)	3m 58s
Test Package (Python 3.11, Django 4.2)	2m 41s
Test Package (Python 3.11, Django 5.0)	2m 57s
Test Package (Python 3.11, Django 5.1)	3m 21s
Test Package (Python 3.11, Django main)	3m 56s
Test Package (Python 3.12, Django 4.2)	2m 35s
Test Package (Python 3.12, Django 5.0)	2m 59s
Test Package (Python 3.12, Django 5.1)	3m 25s
Test Package (Python 3.12, Django main)	4m 9s
Test Package (Python 3.8, Django 4.2)	2m 28s
Test Package (Python 3.9, Django 4.2)	2m 37s
Test successful	1s
	44m 46s

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