Bump docker/build-push-action from 5 to 6 #3049

Job	Run time
Check if database upgrade tests should be run	4s
check_branch	1s
Build Openfire from source (11, zulu)	8m 45s
Build Openfire from source (17, zulu)	8m 38s
Execute Connectivity CI tests	33s
Execute aioxmpp-based CI tests	2m 7s
Test Postgres Upgrades	1m 14s
Test MySQL Upgrades	1m 11s
Execute Smack-based CI tests	41s
Test SQL Server Upgrades	1m 24s
Check if Docker Hub secrets exist	1s
Publish to Maven	0s
Publish to Docker Hub	0s
Test tagged images published to Docker Hub	0s
	24m 39s

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