Bump protobuf from 3.13.0 to 3.15.0 (#104)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.13.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.13.0...v3.15.0)

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

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

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

doc/requirements.txt

@@ -45,7 +45,7 @@ git+https://github.com/XanaduAI/pennylane.git#egg=pennylane
 PennyLane-Lightning==0.23
 Pillow==9.1.0
 pluggy==1.0.0
-protobuf==3.20.0
+protobuf==3.15.0
 PubChemPy==1.0.4
 py==1.11.0
 Pygments==2.11.2

requirements.txt

@@ -32,7 +32,7 @@ git+https://github.com/XanaduAI/pennylane.git#egg=pennylane
 PennyLane-Lightning==0.22.1
 Pillow==9.1.0
 pluggy==1.0.0
-protobuf==3.13.0
+protobuf==3.15.0
 PubChemPy==1.0.4
 py==1.10.0
 pyparsing==2.4.7