Added RabbitMQ 4 commodity

[brooks630]

• What kind of change does this PR introduce (Bug fix, feature, docs update, ...)?
  Feature

• What is the current behavior?
  RabbitMQ 3 is supported as a commodity, however RabbitMQ 4 is not

• What is the new behavior (if this is a feature change)?
  RabbitMQ 4 is available as a commodity.

• Does this PR introduce a breaking change? If so, what actions will users need to take in order to regain compatibility?
  No

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[brooks630]

There are some breaking changes between RabbitMQ 3 and 4; see https://www.rabbitmq.com/docs/whats-new - hence adding it as a new commodity.

[sichapman]

No need for 2.0/3.7 compose fragments as next devenv version will not support them

[sichapman]

I think although postgres treats minor versions as patches, rabbit may treat them as actual feature releases, so our externally managed versions may get left behind by local?

[brooks630]

Would a fix to 4.2 be okay? I'd also like to avoid going the opposite way and having a version that's too far behind locally.

[sichapman]

Might be worth checking with w****s that 4.2 is correct

[sichapman]

but yes if so

[brooks630]

4.2 is fine according to w****s