GHA: Drop Ubuntu 20.04 and other maintenance (2.6)

GHA: Drop Ubuntu 20.04 and other maintenance (2.6)

- Drop Ubuntu 20.04
  GHA runners will go away in April 2025
- Change ubuntu-latest to ubuntu-24.04
  to make sure we are not surprised by
  future changes.
- Update vcpkg digest to latest 33e9c99
- Update github actions to latest

Backport changes:
Sync 2.6 GHA with master GHA by
- pinning action references
- adding Ubuntu 24.04 builds
- updating libressl
- updating ASAN builds to include
  "undefined" checker

Change-Id: I29b68675143988c3304395d9d5ec62289cf519a7
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
(cherry picked from commit c26b2e2c5581ad4e14b737df9178a03d6403a5f7)
Message-Id: <20250212215232.998-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30851.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>