markdownlint also uses node so requires the latest versions

[internalsystemerror]

Q	A
Documentation	no
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

After merging https://github.com/laminas/laminas-continuous-integration-action/pull/155, https://github.com/laminas/laminas-continuous-integration-action/pull/133 should have passed checks, however it appears that I missed the fact that markdownlint has its own docker stage.

This PR adds the nodesource repositories for the latest versions of node to the install-markdownlint stage also, in order to fix that.

[Ocramius]

Thanks @internalsystemerror!