Update README for 21.07 release

Update README and versions for 21.06 release (triton-inference-server…

…#3035)

Remove mention of custom backend (triton-inference-server#2905)

Update README for 21.04 release

Update README and versions for 21.03 release (triton-inference-server…

…#2664)

Specify correct TF2 container tag

Update README for 20.12 release

Update README and versions for 20.11 release

Fix build when --upstream-container-version not specified (triton-inf…

…erence-server#2175)

Update README and versions for 20.09 release