[submodule]: update sonic-sairedis (#6544)

* 20b9573 2021-01-24 | [SAI]: update SAI submodule (#775) (HEAD, origin/master, origin/HEAD) [lguohan]
* 667c33d 2021-01-22 | [syncd] Comparison logic add support to LABEL attribute with higher priority (#764) [Kamil Cudnik]
* aaf5b98 2021-01-22 | [vslib]: Fix missing MACsec Create Port action (#770) [Ze Gan]

Signed-off-by: Guohan Lu <lguohan@gmail.com>