supprted meson version policy clarification

Nobody has claimed this yet.

While updating lua to 5.5 in #2573, the minimal required meson version was updated to `meson_version: '>= 0.63.0'` because the CI was saying that `c_std` cannot be used in subproject `default_options` before `0.63.0`.

However, CI was still complaining, and to me the reason wasn't clear. The readme states

> **Newly added or updated ports in this repository require Meson ≥ 0.56**.

so I thought requiring 0.63.0 is fine.

I had gone through the CI pipeline and found that that there is a check (`test_meson_version_deps`) that only runs if the `TEST_MESON_VERSION_DEPS` env var is defined, which will forcefully set the `meson_version` in the `project()` call to `0.56.0`.

Wouldn't it be clearer to state that

> **Newly added or updated ports in this repository _must have `meson_version: '>= 0.56.0'`_**.

or something similar?

Read the README policy and inspect the test_meson_version_deps check, which is enabled by TEST_MESON_VERSION_DEPS and forces meson_version to 0.56.0. Clarify the README wording so the required meson_version policy matches the check, then verify the relevant CI check still passes.

Written by the indexing model from the issue text.