[firebase-release] Removed change log and reset repo after 3.21.0 release

Author: google-oss-bot

CHANGELOG.md

@@ -1,9 +0,0 @@
-- Adds CPU option and enhances internal data structures (#1077)
-- Add auth blocking handlers (#1080)
-- Add support for secrets in v2 (#1079)
-- Update types for AlertPayloads (#1087)
-- Update AppDistribution `[@type]` (#1088)
-- Update CloudEvent types (#1089)
-- Generate documentation with api-extractor (#1071)
-- Change type info to be inheritance friendly. (#1091)
-- Changes the memory options from MB to MiB and GB to GiB for greater clarity (#1090)