Update prometheus role to last release

Related to !31 (merged).

We want to maintain our dependencies updated and when we try to update the prometheus role up to 2.14.1 an error was raised in the task alerting rules file:

fatal: [monitor.local]: FAILED! => {"changed": false, "checksum": "0b1c1b008fd061938b2d139ec103bd1e72c5da6b", "exit_status": 1, "msg": "failed to validate", "stderr": "  FAILED:\ngroup \"ansible managed alert rules\", rule 13, \"NodeHighNumberConntrackEntriesUsed\": msg=template: __alert_NodeHighNumberConntrackEntriesUsed:1: function \"humanizePercentage\" not defined\n", "stderr_lines": ["  FAILED:", "group \"ansible managed alert rules\", rule 13, \"NodeHighNumberConntrackEntriesUsed\": msg=template: __alert_NodeHighNumberConntrackEntriesUsed:1: function \"humanizePercentage\" not defined"]}