Add Knative v1.21 release schedule (#358)

* Add Knative v1.21 release schedule

* Add next year entry

Author: dsimansk

README.md

@@ -17,10 +17,15 @@ The Knative project releases quarterly, on Tuesday of the 4th week of January, A
 
 | Release | Release Date | Release Leads                                                     | PKG cut    | Unpin repos |
 |---------|--------------|-------------------------------------------------------------------|------------|-------------|
-| v1.18   | 2025-04-22   | TBD                                                               | 2025-04-15 | 2025-04-23  |
 | v1.19   | 2025-07-22   | TBD                                                               | 2025-07-15 | 2025-07-23  |
 | v1.20   | 2025-10-21   | TBD                                                               | 2025-10-14 | 2025-10-22  |
 
+### Release schedule for 2026
+
+| Release | Release Date | Release Leads                                                     | PKG cut    | Unpin repos |
+|---------|--------------|-------------------------------------------------------------------|------------|-------------|
+| v1.21   | 2026-01-20   | TBD                                                               | 2026-01-13 | 2026-01-27  |
+
 
 ## Release leads
 The current pool of Knative release lead volunteers are listed in the [release roster](./ROSTER.md). If you would like to be a Knative release lead, please open a PR to add your name to the list! Please note: Before volunteering to lead a specific release, please look over the [release timeline](TIMELINE.md) to ensure your availability during the various checkpoints for that release is going to be a match.