Remove Alpha notice

[MarcusSorealheis]

Description

Removes Alpha notice as it is out of date.

Type of change

• This change requires a documentation update

Checklist

• Updated documentation if needed
• PR is contained in a single commit, using git amend see some docs

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This change is 

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Name	Status	Preview	Comments	Updated (UTC)
nativelink-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 4, 2024 5:47am

[allada]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: 1 of 1 LGTMs obtained, and pending CI: Analyze (javascript-typescript), Analyze (python), Bazel Dev / ubuntu-22.04, Cargo Dev / macos-13, Cargo Dev / ubuntu-22.04, Local / ubuntu-22.04, Remote / large-ubuntu-22.04, asan / ubuntu-22.04, docker-compose-compiles-nativelink (20.04), docker-compose-compiles-nativelink (22.04), integration-tests (20.04), integration-tests (22.04), macos-13, pre-commit-checks, publish-image, ubuntu-20.04 / stable, ubuntu-22.04, ubuntu-22.04 / stable, windows-2022 / stable, zig-cc ubuntu-20.04, zig-cc ubuntu-22.04

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README.md line 9 at r1 (raw file):

remote executor for systems that communicate using the [Remote execution
protocol](https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto) such as [Bazel](https://bazel.build), [Buck2](https://buck2.build), [Goma](https://chromium.googlesource.com/infra/goma/client/) and
[Reclient](https://github.com/bazelbuild/reclient). NativeLink powers over one  billion requests per month for customers using for the system for their  production workloads.

nit: remove two spaces.

[MarcusSorealheis]

Reviewable status: 2 of 1 LGTMs obtained

[adam-singer]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 2 of 1 LGTMs obtained