[Task] Fix license headers for GitHub SVNs in the repo and declare them in LICENSE/NOTICE

[alaahong]

These issues capture the non-blocking follow-up items raised by the IPMC reviewers on the 2.1.0-incubating IPMC vote thread (https://lists.apache.org/thread/6m0vhnc3zknb58t6rkg3wlpt2bp6rg5r)
Background

The repository tracks GitHub mark icons used in the website frontend:

website/src/pages/team/github.svg
website/static/img/github_icon.svg
These SVGs are derived from GitHub's brand mark. GitHub's brand/mark assets are not under the Apache License and are governed by GitHub's logo guidelines. Bundled as-is in the ASF repository, they lack the required license header and are not declared in LICENSE/NOTICE, violating ASF 3rd-party licensing policy.

Task

Bring the GitHub SVGs into compliance. Depending on the chosen direction:
Replace them with an ASF-compatible GitHub mark substitute, or
If the assets are to be kept, obtain/confirm a compatible license and add a clear LICENSE.header/attribution, then declare them under a new LICENSE/NOTICE entry for "GitHub brand mark" with the exact license text (not a bare link).
If they cannot be licensed, remove them and substitute project-owned icons.

[pjfanning]

If you exclude the files from the source release tar.gz, then they shouldn't be listed in the source release LICENSE/NOTICE.
The source file headers should be fixed.

[alaahong]

PR #1126 fixes this: the two GitHub SVGs are replaced with the CC0 1.0-licensed GitHub mark from Simple Icons (ASF Category-A compatible) and declared in LICENSE with the full CC0 1.0 legal text.