`DOXYGEN_VERSION` does not match what is actually used in CI

The actual [github workflow](https://github.com/zephyrproject-rtos/zephyr/blob/c8d91030f8a4da05cdf6f5e0902cfed94f7e1ee4/.github/workflows/doc-build.yml#L18) environment is different than what is included in the docker images in this repository.

Could this be a mistake?