Remove utf8validator and protolegacy build tag (#7146)

## What changed?
Remove the utf8validator component and stop building with protolegacy.

## Why?
Let protobuf validate all strings are valid UTF-8 as specified.

## How did you test it?
Will go through normal release validation.

## Potential risks
All rpcs and persisted data containing invalid UTF-8 in strings will be
rejected immediately. We've been monitoring for invalid data in Temporal
Cloud for most of a year now and all the bad data is gone.