[R][CI] Disable Rhub GCC 12 and GCC 13 with LTO jobs — images no longer match CRAN

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

The "Rhub GCC 12" and "Rhub GCC 13 with LTO" jobs in r_extra.yml are intended to track CRAN's GCC check environment, but CRAN's GCC-based check flavors now use GCC 15, while the rhub Docker images provide GCC 12 and GCC 13 respectively.

This means both jobs are testing compiler configurations CRAN no longer uses. Since rlang 1.2.0 (released 2026-04-07), the GCC 13 job fails consistently because rlang doesn't compile under GCC 13 with LTO — but it passes fine on CRAN's actual GCC 15 infrastructure.

We should disable both jobs until a matching rhub image is available (see #49738).

References

• CRAN check flavors: https://cran.r-project.org/web/checks/check_flavors.html (GCC 15.x)
• Upstream image issue: ubuntu-gcc12 update gcc? r-hub/containers#98
• Upstream PR (blocked): Add ubuntu-gcc15 container r-hub/containers#99
• GCC 15 PPA now available for Noble: https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test/+packages?field.name_filter=gcc-15
• Example failing run: https://github.com/apache/arrow/actions/runs/24387629892/job/71225235878

Component(s)

R, Continuous Integration

[thisisnic]

Issue resolved by pull request 49739
#49739