Use string for out version value.

[robdimsdale]

• concourse expects map[string]string
• use timestamp so the values are unique and monotonically increasing.

[robdimsdale]

@drnic @longnguyen11288 this is broken as of concourse 0.69.0 and will be explicitly required as of concourse 0.70.0.

Once this is merged, can we also make a final release in https://github.com/cloudfoundry-community/slack-notification-resource-boshrelease