Cleanup repository structure

[kukushechkin]

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.