ci: pin ruff-action to 0.15.16

The Ruff Action had no version pin, so it always installs "latest" ruff.
ruff 0.16.0 fails CI on files unrelated to any given PR's diff — it added
CPY001 (missing copyright notice) and started flagging PLR0917
(too-many-positional-arguments) under this repo's `select = ["ALL"]"
config, neither of which the existing codebase satisfies. This makes CI
fail nondeterministically depending only on when a PR happens to trigger
a fresh ruff install, not on the PR's actual changes.

Pinning to 0.15.16 (the version used by the last known-green CI run,
commit f20d445) restores a clean, reproducible `ruff check`.

Author: johanzander

.github/workflows/ruff.yml

@@ -10,4 +10,5 @@ jobs:
       - uses: actions/checkout@v4
       - uses: astral-sh/ruff-action@v3
         with:
-          src: "./growattServer"
+          src: "./growattServer"
+          version: "0.15.16"