CI jobs for Camel 3.21 and remove 3.18

[Claus Ibsen]

Camel 3.18 is EOL and we can remove CI jobs for it

Camel 3.21 will be released soon, so we should setup CI jobs for this
branch.


-- 
Claus Ibsen
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@davsclaus
Camel in Action 2: https://www.manning.com/ibsen2