Bump proto-plus from 1.25.0 to 1.26.1 in /requirements

Bumps [proto-plus](https://github.com/googleapis/proto-plus-python) from 1.25.0 to 1.26.1.
- [Release notes](https://github.com/googleapis/proto-plus-python/releases)
- [Changelog](https://github.com/googleapis/proto-plus-python/blob/main/CHANGELOG.md)
- [Commits](googleapis/proto-plus-python@v1.25.0...v1.26.1)

---
updated-dependencies:
- dependency-name: proto-plus
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements/prod.txt

@@ -905,9 +905,9 @@ google-crc32c==1.6.0 \
 google-resumable-media==2.7.2 \
     --hash=sha256:3ce7551e9fe6d99e9a126101d2536612bb73486721951e9562fee0f90c6ababa \
     --hash=sha256:5280aed4629f2b60b847b0d42f9857fd4935c11af266744df33d8074cae92fe0
-proto-plus==1.25.0 \
-    --hash=sha256:c91fc4a65074ade8e458e95ef8bac34d4008daa7cce4a12d6707066fca648961 \
-    --hash=sha256:fbb17f57f7bd05a68b7707e745e26528b0b3c34e378db91eef93912c54982d91
+proto-plus==1.26.1 \
+    --hash=sha256:13285478c2dcf2abb829db158e1047e2f1e8d63a077d94263c2b88b043c75a66 \
+    --hash=sha256:21a515a4c4c0088a773899e23c7bbade3d18f9c66c73edd4c7ee3816bc96a012
 protobuf==4.25.6 \
     --hash=sha256:07972021c8e30b870cfc0863409d033af940213e0e7f64e27fe017b929d2c9f7 \
     --hash=sha256:3f3b0b39db04b509859361ac9bca65a265fe9342e6b9406eda58029f5b1d10b2 \