Redmine

Hello,

I have update redmine with 3.4.12 (stack bitnami VM VMWARE)
The installed version of ruby seems to be ruby 2.5.7-p206 but i want to update to 2.6 (for one of my new plugin)
After installing ruby 2.6 with rvm :
ruby -v ==> ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]

But on Redmine information page, i found :
Redmine Version : 3.4.12.stable
Ruby version : 2.5.7-p206
Rails Version : 4.2.11.1

I'm easily working with debian usually but not with gems

I don't understand why Ruby version on redmine is not 2.6 ..??

Thanks for your Help :)