Simplify CI setup

[Alexey Palazhchenko]

• Move all example repositories commands to scripts. For example, this should be in the dotnet-example-plain.sh file.
• Merge SHA-1 and SHA-256 configurations scripts, name them like java-example-scram (drop the auth part, too).
• Update CI configuration.

[b1ron]

@AlekSi I assume we can remove all files that test PLAIN?

[Alexey Palazhchenko]

For example, this should be in the dotnet-example-plain.sh

I'm not sure how you read it as "we should remove PLAIN"