[wip][ci] Use system-builder:2.4

[hallelujah]

Closes THREESCALE-2062

• make tests pass first
• Keep 2.3 build but schedule a nightly build for 2.4
• Extract what can be extracted to master

[codecov]

Codecov Report

Merging #790 into master will decrease coverage by 5.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #790      +/-   ##
==========================================
- Coverage   92.89%   87.85%   -5.05%     
==========================================
  Files        2414     2096     -318     
  Lines       78237    65312   -12925     
==========================================
- Hits        72682    57380   -15302     
- Misses       5555     7932    +2377

Impacted Files	Coverage Δ
...ib/three_scale/spam_protection/checks/timestamp.rb	97.95% <100%> (ø)	⬆️
test/unit/array_test.rb	100% <100%> (ø)	⬆️
...pp/controllers/buyers/impersonations_controller.rb	15.38% <0%> (-84.62%)	⬇️
...ollers/provider/admin/api_docs/specs_controller.rb	20% <0%> (-80%)	⬇️
app/controllers/heroku/sessions_controller.rb	22.22% <0%> (-77.78%)	⬇️
...p/controllers/admin/api/heroku_proxy_controller.rb	25% <0%> (-75%)	⬇️
app/helpers/buyers/fields_definitions_helper.rb	25% <0%> (-75%)	⬇️
app/controllers/partners/providers_controller.rb	25% <0%> (-75%)	⬇️
app/helpers/help_bubbles_helper.rb	28.57% <0%> (-71.43%)	⬇️
app/controllers/partners/users_controller.rb	26.66% <0%> (-70%)	⬇️
... and 695 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d050d81...c62ae04. Read the comment docs.

[hallelujah]

So @3scale/system I do not know if we should make it run on 2.3 and 2.4 ...

On one hand, it would be no work for @3scale/operations but on the other hand we are going to keep an already dead version

Most of the commits here can be extracted first as single PR into the master branch. I will do that on Monday.

[hallelujah]

Closing as superseeded by #814