Add Migration Guide from Spring Security SAML Extension #8340

Job	Run time
Check Samples	10m 6s
Build (ubuntu-latest, 17) / Build	2m 8s
Build (windows-latest, 17) / Build	5m 25s
Test Against Snapshots (17, 17) / Test	7m 49s
Test Against Snapshots (21-ea, 21) / Test	7m 43s
Deploy Schema / Deploy Schema	1m 3s
Deploy Docs / Deploy Docs	1m 33s
Deploy Artifacts / Deploy Artifacts	2m 1s
Perform Release / Perform Release	0s
Send Notification	4s
	37m 52s

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