resume bumping vcpkg version and releasing a v1 series based on ubuntu-bionic for older glibc compat

[qrkourier]

We stopped releasing the bionic-based v1 series when we started basing the v2 build on focal, but now we need an old glibc builder image with up-to-date vcpkg.

The vcpkg updates are already on autopilot, but they're only feeding in to new releases of focal-based v2.

Change the workflow to feed vcpkg updates to both bionic-based v1 releases and focal-based v2 releases, creating a separate PR for each, and ensuring release integrity of the parallel series.