chore(deps): update dependency com_google_protobuf to v3.21.0 (#481)

[![Mend Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [com_google_protobuf](https://togithub.com/protocolbuffers/protobuf) | http_archive | minor | `v3.20.1` -> `v3.21.0` |

---

### Release Notes

<details>
<summary>protocolbuffers/protobuf</summary>

### [`v3.21.0`](https://togithub.com/protocolbuffers/protobuf/compare/v3.20.1...v3.21.0)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.20.1...v3.21.0)

</details>

---

### Configuration

📅 **Schedule**: At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, click this checkbox.

---

This PR has been generated by [Mend Renovate](https://www.mend.io/free-developer-tools/renovate/). View repository job log [here](https://app.renovatebot.com/dashboard#github/googleapis/gapic-generator-php).

bazel_example/WORKSPACE.bazel

@@ -16,9 +16,9 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
     name = "com_google_protobuf",
-    sha256 = "662879e41508a5ecce3be2c65563a8fac3301a48adef3113913ec4010f405a33",
-    strip_prefix = "protobuf-3.20.1",
-    urls = ["https://github.com/protocolbuffers/protobuf/archive/v3.20.1.zip"],
+    sha256 = "05e368b4fe0a71c096b68f7e2f0c81ba61743d06baa858cf6f00ce78959c12c6",
+    strip_prefix = "protobuf-3.21.0",
+    urls = ["https://github.com/protocolbuffers/protobuf/archive/v3.21.0.zip"],
 )
 
 load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")