fix: Regenerate v0.20.0 protos

[arielvalentin]

Description

This change does a few things:

1. Adds a guide for maintainers to update protos
2. Updates the rake task to regenerate code from protos using a clean directory.
   1. This ensures any changes upstream that deletes protos are applied to this repo
3. Updates the rake task to fast fail when it encounters an error, e.g. protoc is not installed

What's an error generating code look like?

@arielvalentin ➜ /workspaces/opentelemetry-ruby/exporter/otlp (refactor-proto-script) $ bundle exec rake protobuf:generate
Cloning into 'opentelemetry-proto'...
remote: Enumerating objects: 2211, done.
remote: Counting objects: 100% (641/641), done.
remote: Compressing objects: 100% (238/238), done.
remote: Total 2211 (delta 449), reused 503 (delta 342), pack-reused 1570
Receiving objects: 100% (2211/2211), 671.91 KiB | 2.76 MiB/s, done.
Resolving deltas: 100% (906/906), done.
Note: switching to '1608f92cf08119f9aec237c910b200d1317ec696'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

rake aborted!
Errno::ENOENT: No such file or directory - protoc
/workspaces/opentelemetry-ruby/exporter/otlp/Rakefile:45:in `system'
/workspaces/opentelemetry-ruby/exporter/otlp/Rakefile:45:in `block (3 levels) in <top (required)>'
/workspaces/opentelemetry-ruby/exporter/otlp/Rakefile:44:in `each'
/workspaces/opentelemetry-ruby/exporter/otlp/Rakefile:44:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-3.2.2/gems/rake-12.3.3/exe/rake:27:in `<top (required)>'
/usr/local/rvm/gems/ruby-3.2.2/bin/ruby_executable_hooks:22:in `eval'
/usr/local/rvm/gems/ruby-3.2.2/bin/ruby_executable_hooks:22:in `<main>'
Tasks: TOP => protobuf:generate
(See full trace by running task with --trace)