Refactor: Replace LICENSE sha256 with SPDX-License-Identifier

[filmil]

Replaced all instances of LICENSE sha256 (and sha265 typos) with SPDX-License-Identifier: Apache-2.0. I also moved these license identifiers to the very first line of each modified source file, and ran the documentation generation scripts (bazel run @@//cmd/lit2md:update_md and bazel run @@//:update) so that the generated .md docs are consistent with the source files. All bazel test //... tests now pass.

---

PR created automatically by Jules for task 2334199502229155835 started by @filmil

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.