Unify repos layout across the ecosystem:

1. Remove `CONTRIBUTORS.md` and mentions of it.
2. Remove `CODE_OF_CONDUCT.md` and mentions of it.
3. From `CONTRIBUTING.md` remove "Writing a Patch" and "Commit Message
Template" sections.
4. Remove `.mailmap` and mentions of it.
5. Remove `dev/git.commit.template` and mentions of it.
6. Remove `SECURITY.md`, if it is referenced somewhere, change it to
reference to the org security file
"https://github.com/apple/swift-log?tab=security-ov-file".
7. Remove copyright header from `Package.swift` and version-specific
package manifests.
8. Add SPI shields to the `README.md`.
9. Add "How to contribute your work" section in `CONTRIBUTING.md`.
10. Add "Add Automated release process" section in `CONTRIBUTING.md`.