Update qsim from 0.13.3 to 0.21.0 (#982)

## Description

Updates the qsim dependency from version 0.13.3 to 0.21.0.

Fixes #830

## Changes

- Updated `WORKSPACE` file to use qsim v0.21.0
- Updated SHA256 hash for the new version
- Updated `strip_prefix` to match the new version

## Why

The previous version of qsim (0.13.3) is outdated. Version 0.21.0
includes bug fixes and improvements, including support for matrix gates
(see quantumlib/qsim#660).

## Testing

- [x] Build passes: `bazel build //tensorflow_quantum/...`
- [x] Tests pass: `bazel test //tensorflow_quantum/...`

Author: saksham-1304

WORKSPACE

@@ -68,12 +68,12 @@ cc_library(
 
 http_archive(
     name = "qsim",
-    sha256 = "b9c1eba09a885a938b5e73dfc2e02f5231cf3b01d899415caa24769346a731d5",
+    sha256 = "720eeb97298819e00bbb218b8b58fcebbbc1e1708233598fdffeef0b97339617",
     # patches = [
     #     "//third_party/tf:qsim.patch",
     # ],
-    strip_prefix = "qsim-0.13.3",
-    urls = ["https://github.com/quantumlib/qsim/archive/refs/tags/v0.13.3.zip"],
+    strip_prefix = "qsim-0.21.0",
+    urls = ["https://github.com/quantumlib/qsim/archive/refs/tags/v0.21.0.zip"],
 )