ci: use more recent linux32 image

The Xenial image we're using was released more than 8 years ago. This is
a problem for using some recent GitHub Actions scripts, as they require
Node.js 20, and all of the binaries they ship need glibc 2.28 or later.
We're not using them yet, but moving forward prepares us for a future
patch which will.

Xenial was actually the last official 32-bit Ubuntu release, but you can
still find i386 images for more recent releases. This patch uses Focal,
which was released in 2020 (and is the oldest one with glibc 2.28).

There are two small downsides here:

  - while Xenial is pretty old, it is still in LTS support until April
    2026. So there's probably some value in testing with such an old
    system, and we're losing that.

  - there are no i386 subversion packages in the Focal repository. So we
    won't be able to test that (OTOH, we had never tested it until the
    previous patch which unified the 32/64-bit dependency code).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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