Update MongoDB version in pack GHA workflow

[arm4b]

Apparently, we use mongo:3.4 in CircleCI configs across our Exchange fleet. This version is deprecated and not supported by stackstorm anymore.

Example:
https://github.com/StackStorm-Exchange/stackstorm-ansible/blob/35503b3cf6009856c9cba08f9584669742593fcd/.circleci/config.yml#L8

Instead switch to the latest stackstorm-supported MongoDB version.

[cognifloyd]

Now that we're using GHA, this needs to be updated in these places:

• https://github.com/StackStorm-Exchange/ci/blob/master/.github/workflows/pack-build_and_test.yaml#L71
• https://github.com/StackStorm-Exchange/exchange-incubator/blob/master/.github/workflows/build_and_test.yaml#L65
• https://github.com/StackStorm-Exchange/stackstorm-vault/blob/master/.github/workflows/build_and_test.yaml#L102

That will update it for the entire exchange!