Update readme to Juju 3.x #1745

Job	Run time
Lint	29s
Unit tests	3m 23s
sync-docs / Open PR with docs changes	3m 4s
Build charms (tests/integration/app-charm) / Collect bases for charm \| tests/integration/app-charm	20s
Build charms (.) / Collect bases for charm \| .	14s
Build charms (.) / Build charm \| base #0	3m 41s
Build charms (tests/integration/app-charm) / Build charm \| base #0	2m 39s
integration-charm	46m 28s
integration-provider	45m 41s
integration-scaling	22m 6s
integration-password-rotation	26m 51s
integration-tls	39m 47s
integration-upgrade	21m 18s
integration-balancer-single	58m 30s
integration-balancer-multi	1h 3m 53s
integration-kraft-single	9m 28s
integration-kraft-multi	12m 45s
${{ matrix.tox-environments }}	0s
	6h 0m 37s

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