v3.0: start 3.0.5

[ralfhandl]

Just in case we want to start a 3.0.5 ...

• src/oas.md copied from versions/3.0.4.md
• Version headline and revision history in Appendix A adjusted: ef6932dd1a04c179111cae958533ddbfe5832923

Merge into v3.0-dev-wip before renaming it to v3.0-dev, or change the base branch.

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• [x] no schema changes are needed for this pull request