Update file semconv/template.j2, replace required by requirementlevel

[Petrie]

what happened

When I exec below cmd for regenerate semconv/v1.12.0

$ export TAG="v1.12.0"
$ export OTEL_SPEC_REPO="/path/to/opentelemetry-specification/"
$ git -C "$OTEL_SPEC_REPO" checkout "tags/$TAG"
$ make semconv-generate

There will be a jinja2.exceptions.UndefinedError error as follow:

Traceback (most recent call last):
  File "$HOME/cncf/opentelemetry/build-tools/semantic-conventions/src/opentelemetry/semconv/main.py", line 241, in <module>
    main()
  File "$HOME/cncf/opentelemetry/build-tools/semantic-conventions/src/opentelemetry/semconv/main.py", line 75, in main
    renderer.render(semconv, args.template, args.output, args.pattern)
  File "$HOME/cncf/opentelemetry/build-tools/semantic-conventions/src/opentelemetry/semconv/templating/code.py", line 251, in render
    template.stream(data).dump(output_file)
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 1618, in dump
    fp.writelines(iterable)
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 1613, in <genexpr>
    iterable = (x.encode(encoding, errors) for x in self)  # type: ignore
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 1662, in __next__
    return self._next()  # type: ignore
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 1354, in generate
    yield self.environment.handle_exception()
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "$HOME/cncf/opentelemetry/opentelemetry-go/semconv/template.j2", line 62, in top-level template code
    // {{ godoc(attr) | wordwrap | indent(3) | replace("   ", "\t// ") | replace("// //", "//") }}
  File "/usr/local/lib/python3.9/site-packages/jinja2/runtime.py", line 777, in _invoke
    rv = self._func(*arguments)
  File "$HOME/cncf/opentelemetry/opentelemetry-go/semconv/template.j2", line 19, in template
    {%- if attr.required == Required.ALWAYS %}
  File "/usr/local/lib/python3.9/site-packages/jinja2/environment.py", line 485, in getattr
    return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'Required' is undefined

And why

This error caused by the update in PR #https://github.com/open-telemetry/build-tools/pull/92/files , Class Required is replaced by RequirementLevel .
More details https://github.com/open-telemetry/build-tools/pull/92/files#diff-f62a4dc611090cd2aee181fcd6522c010708dd98d5cf262c4f06ef61a519a25aL31

What we should do

As the attribute required was totally replaced by requirementlevel in open-telemetry/opentelemetry-specification#2594. we should update our semconv/template.j2 to adopt it.