Swap in protovalidate instead of protoc-gen-validate

[tstirrat15]

Description

protoc-gen-validate is incompatible with python 3.13, which is a python version that we want to support. This swaps it out for protovalidate, which is the successor.

Changes

• Remove python 3.8 from support matrix as it's EOL
• Regenerate client based on branch that includes protovalidate annotations
• Remove protoc-gen-validate
• Add protovalidate as a runtime dependency
• Remove old validate package
• Add test

Testing

Review. See that tests pass and that new tests make sense.