Cluster workers upgraded to Ubuntu 26.04

All Linux cluster workers upgraded from Ubuntu 24.04 to 26.04 (kernel 6.8 → 7.0):

• linux-x86_64: asteria, bremusa, clete, doris, eumache, laodoke, odawa, phoebe, toxis
• linux-arm64: ainia, kydoime, molpadia, okypous, ocaml-1, ocaml-2
• linux-ppc64: orithia, scyleia
• linux-s390x: s390x-worker-01

riscv-bm-01–04 remain on Ubuntu 24.04; the hardware does not support 26.04.