Move reference projects builds to Jenkins

[NicoPrediger]

Due to some changes with travis-ci, we want to move the reference project build matrix to Jenkins. We already have two non-JIRO agents which would be perfect for that job.

[cdietrich]

move to 2.25
see also xtext/xtext-reference-projects#150

[cdietrich]

as an emergency measure for master, i am reparing an github action

[cdietrich]

had to turn of travis. was failing all the time due maven central connection problems

[cdietrich]

move to 2.26

[cdietrich]

this is really a problem with e.g. to fix #1976

[LorenzoBettini]

Isn't it better to stay on GitHub Actions?

[cdietrich]

@LorenzoBettini this is two fold

• Github Actions is unstable (have to rerun regularly, one can rerun all jobs only and not single failed runs)
• We want to build all projects back to Xtext 2.12. So we will run into limitations by github actions.

[LorenzoBettini]

Not that JIRO is known for its stability... ;)

anyway, if we hit limitations, then it's better to switch

[cdietrich]

done