Replace ruby:2.0/2.1 in examples and test cases with ruby:2.6

What does this MR do?

Replaces ruby:2.0/2.1 in examples and test cases with ruby:2.6.

cf. !1748 (merged) for docs

Why was this MR needed?

Obviously strings in test cases are arbitrary but these changes would help documentation maintainers keep docs up to date.

Are there points in the code the reviewer needs to double check?

n/a

Does this MR meet the acceptance criteria?

• [n/a] Documentation created/updated --> See !1748 (merged)
• [n/a] Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

n/a