README.md: drop experimental for Linux/macOS [ci skip]

Preparing for curl 8.10.0, where certs are expected to work out of
the box, and `RANDOM_FILE` is dropped. Also for any next 8.9.1 build
where some cross-build issues have been ironed out recently.

README.md

@@ -44,7 +44,7 @@ SPDX-License-Identifier: CC-BY-SA-4.0
   Using reproducible OS image
   [`debian:testing-slim`](https://github.com/debuerreotype/docker-debian-artifacts/tree/dist-amd64/testing/slim)
   via [Docker](https://hub.docker.com/_/debian/). macOS built via GHA.
-- Linux and macOS builds are experimental and *not* official curl builds.
+- Linux and macOS builds are *not* official curl builds.
 - We verify components using SHA-256, and PGP where provided.
 - We build [reproducible](https://reproducible-builds.org/) binaries,
   producing the same hash given the same input sources and C compiler.