TOOL-30781 point linux-pkg's os-upgrade branch at the resolute suite

[prakashsurya]

Problem

linux-pkg's os-upgrade branch still defaults UBUNTU_DISTRIBUTION to noble, so every linux-pkg/os-upgrade job points apt at a suite the branch's mirror does not carry.

The mirror is no longer what's blocking. os-upgrade/latest/ promoted on 2026-08-08 05:09 UTC (snapshot 8411ab44) and publishes resolute, and only resolute:

ubuntu/dists/   resolute  resolute-backports  resolute-security  resolute-updates
ppas/dists/     resolute  resolute-pgdg  stable

with ubuntu/dists/resolute/Release reporting Codename: resolute / Version: 26.04. Since there are no noble suites under this branch, configure_apt_sources() writes deb <mirror> noble ... lines that resolve to nothing and the jobs die before they get as far as git:

E: The repository '.../os-upgrade/latest/ubuntu noble-backports Release' does not have a Release file.
Error: failed command 'sudo apt-get update'

The generic kernel lookup is pinned the same way; get_kernel_version_for_platform_from_apt() asks for linux-image-generic-hwe-24.04. That one does not hard-fail, which is what makes it worth catching now: resolute still publishes an hwe-24.04 metapackage, and it depends on the same 7.0.0-14.14 kernel as hwe-26.04, so the stale pin resolves to the right kernel by accident and would not have announced itself for the rest of the cycle.

Solution

The solution taken in this PR is to bump both pointers:

• UBUNTU_DISTRIBUTION default noble -> resolute in lib/common.sh. It stays environment-overridable (${UBUNTU_DISTRIBUTION:-resolute}, from TOOL-30719), so a build can still be aimed at another suite without editing the branch.
• the generic HWE metapackage linux-image-generic-hwe-24.04 -> -hwe-26.04.

This targets os-upgrade rather than develop, since it cannot land on develop until the Phase 4 cutover. 24.04 analog: DLPX-91976 / #322.

The branch was re-reset to develop (01b2d791e -> fef16a859) before this change, since it had drifted behind again and was missing #408, #410, TOOL-30738 and #405. That reset is also what drops the 9999.0.0.0 mirror-host sentinel, so there is nothing separate to do for it here.

Testing Done

• make check (shellcheck + shfmt) passes.
• The default resolves as intended and the environment override still wins:

(unset)                     -> resolute
UBUNTU_DISTRIBUTION=noble   -> noble

• Checked against the promoted mirror that every suite the new default asks for is actually published: resolute{,-updates,-security,-backports} under ubuntu/, and resolute + resolute-pgdg + stable under ppas/.
• Confirmed linux-image-generic-hwe-26.04 exists in resolute (7.0.0-14.14 in resolute, 7.0.0-29.29 in resolute-updates), so the new pin resolves.

No package build has been run against this yet. build-packages on os-upgrade is the next step and is where the real fallout is expected to show up; the intent is to fix-forward on the branch from there.