Tests

Update GH runners: use juju 3.1.5 for CI #728

Job	Run time
Unit tests	2m 33s
Lint	17s
Check libraries	28s
Build charms / Get workflow version / Get workflow version	3s
Build charms / Collect charms	20s
Build charms / Build mongodb charm \| 22.04	2m 22s
Build charms / Build application charm \| 20.04	2m 6s
charm-integration	11m 34s
ha-integration	52m 2s
relation-integration	24m 20s
legacy-integration	20m 56s
tls-integration	29m 7s
backup-integration	1h 4m 29s
metric-integration	18m 55s
	3h 49m 32s

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