Bump proto-plus from 1.15.0 to 1.18.0 in /requirements (#16761)

Bumps [proto-plus](https://github.com/googleapis/proto-plus-python) from 1.15.0 to 1.18.0.
- [Release notes](https://github.com/googleapis/proto-plus-python/releases)
- [Changelog](https://github.com/googleapis/proto-plus-python/blob/master/CHANGELOG.md)
- [Commits](googleapis/proto-plus-python@v1.15.0...v1.18.0)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/prod.txt

@@ -492,9 +492,9 @@ google-cloud-core==1.6.0 \
 google-resumable-media==1.2.0 \
     --hash=sha256:dbe670cd7f02f3586705fd5a108c8ab8552fa36a1cad8afbc5a54e982cf34f0c \
     --hash=sha256:ee98b1921e5bda94867a08c864e55b4763d63887664f49ee1c231988f56b9d43
-proto-plus==1.15.0 \
-    --hash=sha256:41c8bd85252ee75c029073ff162e9ef6dffcda552d720152b49fa9d328784a65 \
-    --hash=sha256:c66f7573ea5947ec84bf3bb169e28ebc00c5ea468da3f4bf5827adb4b773ae4a
+proto-plus==1.18.0 \
+    --hash=sha256:18f43cdeb66e326e4392a0f20662f3e826f0a35489780510a9d3dc1d8135ce30 \
+    --hash=sha256:bae25ef9e7d4290802d56ec81faba0a7cfe58843276eb15b320677b5b8b3f63d
 protobuf==3.15.6 \
     --hash=sha256:1771ef20e88759c4d81db213e89b7a1fc53937968e12af6603c658ee4bcbfa38 \
     --hash=sha256:1a66261a402d05c8ad8c1fde8631837307bf8d7e7740a4f3941fc3277c2e1528 \