fix: Deprecated warning for Kernel#Namespace()

[rhiroe]

Receive a deprecation warning for Kernel#Namespace() in RBS 3.8 or later.

$ bundle exec rbs --version
rbs 3.8.1

$ bundle exec rake test
Kernel#Namespace() is deprecated. Use RBS::Namespace.parse instead.

Use RBS::Namespace.parse instead of Kernel#Namespace().

I have verified that the test passes on RBS 3.5, 3.6, 3.7, and 3.8.

[tk0miya]

+1 for Namespace.

On the other hand, we need to update the dependency to use RBS::TypeName.parse because it was added since v3.8.0.
ruby/rbs#2123