Update release checklist

[lorentey]

Add two steps, about pre-announcing nontrivial releases and about updating the dependency configuration in the Swift toolchain.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

macOS testing will fail due to rdar://117169742. (Actually it won't, the main branch happens to work.) This is fine. 🔥