chore: pin the next dev beta to 5.3.1-beta.1

Retargets the pending dev beta release from 5.2.0-beta.5 to 5.3.1-beta.1.

The beta line is still numbered off 5.2.0 because the stable 5.3.0 release never
completed: the promote PR (#108) merged with `Release-As: 5.3.0`, but the
release-please PR it generated (#109) is still open, so no v5.3.0 tag exists and
`.release-please-manifest-beta.json` sat at 5.2.0-beta.4. Release-please
therefore proposed 5.2.0-beta.5 (#115) for the flow-discovery fix in #114, which
is behind the stable line rather than ahead of it.

The stable line is 5.3.0, so the next beta belongs on the 5.3.1 patch series.

This commit is intentionally empty — the payload is the footer below, which
release-please reads to pin the exact version of the next dev release. It
applies once and needs no follow-up cleanup, unlike a `release-as` key in
release-please-config-beta.json, which would pin every subsequent dev release
until removed.

Release-As: 5.3.1-beta.1

Author: finalerock44