docs: update demos with latest changes to EC2 plugin and globalLibraries

[colshine1]

update demos with latest changes:

• EC2 - breaking changes to Deprecate boolean associatePublicIp and add AssociateIpStrategy: Deprecate boolean associatePublicIp and add AssociateIpStrategy enum for IP assignment control  ec2-plugin#1148
• globalLibraries - git is replaced with gitSource

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[timja]

The demos are mostly tested see https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/demos/README.adoc

You'll need to update the ec2 plugin and git plugin in the integrations pom.xml

[ZeBidule]

The correct key is associateIPStrategy and not associateIpStrategy

[timja]

I've fixed up the remaining bits, I think this should do it, although I haven't run it locally