7.6 support

[pebrc]

Test and support Elasticsearch version 7.6. I did a first test with a recent snapshot build and
#2177 came out of that. But there are other changes we know of already that we have to support:

• Support "enterprise" license types elasticsearch#49223 support enterprise license type. This will make it necessary in our license selection logic to prefer stack level enterprise licenses over platinum licenses when both are contained in a orchestration level enterprise license.
• Add setting to restrict license types elasticsearch#49418 support xpack.license.upload.types this should be set to trial, enterprise for ECK managed clusters as of version 7.6

[tvernum]

xpack.license.upload.types this should be set to basic, trial, enterprise

It actually needs to be trial, enterprise.
You are never allowed to upload basic licenses, so the latest version of that PR prevents you from including it in the allowed types.