docs(rust-toolchain): add note regarding ci msrv update

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>

rust-toolchain.toml

@@ -1,6 +1,7 @@
 # SPDX-FileCopyrightText: 2024 Christina Sørensen
 # SPDX-License-Identifier: EUPL-1.2
 [toolchain]
+# NOTE: don't forget to update the unit-tests workflow when changing this
 channel = "1.78"
 components = [
   "rustfmt",