[Submodules] Bump plugins/CustomVariables in the all-submodules group (#23170)

Bumps the all-submodules group with 1 update: [plugins/CustomVariables](https://github.com/matomo-org/plugin-CustomVariables).


Updates `plugins/CustomVariables` from `f29c470` to `51f45a0`
- [Release notes](https://github.com/matomo-org/plugin-CustomVariables/releases)
- [Commits](matomo-org/plugin-CustomVariables@f29c470...51f45a0)

---
updated-dependencies:
- dependency-name: plugins/CustomVariables
  dependency-type: direct:production
  dependency-group: all-submodules
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]