dependabot rebase jumped the tracks

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Package ecosystem

Maven

### Package manager version

_No response_

### Language version

Java 11, 14, 17

### Manifest location and content before the Dependabot update

Project: https://github.com/SpiNNakerManchester/JavaSpiNNaker
Manifest: `/pom.xml`

### dependabot.yml content

https://github.com/SpiNNakerManchester/JavaSpiNNaker/blob/master/.github/dependabot.yml

### Updated dependency

Spring Boot, 2.7.14 → 2.7.15 / 3.1.3

### What you expected to see, versus what you actually saw

Dependabot submitted a PR to update Spring Boot from 2.7.14 to 2.7.15. Due to other things going on (permission issue), the PR failed to build automatically along with multiple others submitted at the same time. I fixed that problem and told Dependabot to `@dependabot rebase` to pick up the fix. So far so ordinary.

But the rebased version was no longer a change from 2.7.14 to 2.7.15 *but rather to **3.1.3**,* a different major version with rather a lot of significant changes that made many aspects of the build fail. (I have a branch where this version is in use, but can't update my primary branch to it yet due to messy deployment restrictions.) This is not what I'd expect from a rebase. I would *expect* something more like a classic `git rebase`.

It was easy to work around once I saw what it'd unexpectedly done, but very much not as expected.

### Native package manager behavior

_No response_

### Images of the diff or a link to the PR, issue, or logs

* https://github.com/SpiNNakerManchester/JavaSpiNNaker/pull/1014

### Smallest manifest that reproduces the issue

_No response_

Use the linked JavaSpiNNaker pull request as the reproduction case, starting with its /pom.xml and .github/dependabot.yml. Trace the Maven updater's handling of @dependabot rebase after the original update PR failed, and compare the requested 2.7.15 update with the generated 3.1.3 update. Done means rebasing preserves the intended dependency target and regression coverage captures this case.

Written by the indexing model from the issue text.