Bump Ruby to 4.0

The bosh integration image used in the pipeline has bumped to 4 and
otherwise we get errors like:

Failure/Error: expect(RUBY_VERSION.split(".")[0,1]).to eq(PROJECT_RUBY_VERSION.split(".")[0,1])

     expected: ["3"]
          got: ["4"]

     (compared using ==)
   # ./spec/spec_helper.rb:163:in 'block (2 levels) in <top (required)>'

ai-assisted=yes
[TNZ-88995]

Author: selzoc

src/bosh_aws_cpi/.ruby-version

@@ -1 +1 @@
-3.4
+4.0