build(deps): bump protobuf from 3.15.0 to 3.18.3 in /udf/agent/py (#2720)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.0 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.0...v3.18.3)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

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

Author: dependabot[bot]

udf/agent/py/setup.py

@@ -14,7 +14,7 @@
         'kapacitor.udf',
     ],
     install_requires=[
-        "protobuf==3.15.0",
+        "protobuf==3.18.3",
     ],
     maintainer_email="support@influxdb.com",
     license="MIT",