Update and rename NNNN-calendar-improvements.md to 0001-calendar-improvements.md (swiftlang#354)

- Assign proposal number
- Update review manager and status

Author: itingliu

Proposals/NNNN-calendar-improvements.md renamed to Proposals/0001-calendar-improvements.md

@@ -1,9 +1,9 @@
 # Calendar Sequence Enumeration
 
-* Proposal: [SF-NNNN](NNNN-calendar-improvements.md)
+* Proposal: [SF-0001](0001-calendar-improvements.md)
 * Authors: [Tony Parker](https://github.com/parkera)
-* Review Manager: TBD
-* Status: **Awaiting review**
+* Review Manager: [Tina Liu](https://github.com/itingliu)
+* Status: **Active review: Dec 18, 2023...Dec 26, 2023**
 * Implementation: [Pull Request](https://github.com/apple/swift-foundation/pull/322)
 * Review: [Pitch](https://forums.swift.org/t/pitch-calendar-sequence-enumeration/68521)