doc: write release notes for Go 1.21

[thanm]

This is the tracking issue for writing the Go 1.21 Release Notes. The version at tip can be viewed at https://tip.golang.org/doc/go1.21.

When the Go 1.21 Release Notes are complete, this should be closed, and a similar issue should be made for the next release. Previous issues are #54202, #51400, #47694, #44513, etc.

[gopherbot]

Change https://go.dev/cl/470296 mentions this issue: doc: start draft Go 1.21 release notes

[gopherbot]

Change https://go.dev/cl/495775 mentions this issue: doc/go1.21: convert .html to .md

[gopherbot]

Change https://go.dev/cl/496116 mentions this issue: doc: release notes for my humble contributions to go1.21

[gopherbot]

Change https://go.dev/cl/496316 mentions this issue: doc: add TODOs to go1.21

[gopherbot]

Change https://go.dev/cl/496194 mentions this issue: doc: add slog to release notes

[gopherbot]

Change https://go.dev/cl/497095 mentions this issue: doc: write various runtime-related release notes