update debian changelog for release v0.21.0

update debian changelog for release v0.21.0

  * Support for kernel up to 5.13
  * support for debug information from libdebuginfod
  * finished support for map elements items_*_batch() APIs
  * add atomic_increment() API
  * support attach_func() and detach_func() in python
  * fix displaying PID instead of TID for many tools
  * new tools: kvmexit.py
  * new libbpf-tools: gethostlatency, statsnoop, fsdist and solisten
  * fix tools ttysnoop/readahead for newer kernels
  * doc update and bug fixes

Signed-off-by: Yonghong Song <yhs@fb.com>