chore(java-iam-policy): update name pretty

[zhumin8]

Bring in change made in #13378 for iam-policy name to config and generate.
run these commands locally:

V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
go run github.com/googleapis/librarian/cmd/librarian@${V} generate iam-policy

Fixes googleapis/librarian#6336

[gemini-code-assist]

Code Review

This pull request updates the pretty name and documentation references for the IAM Policy service from "IAM" to "Google Cloud IAM Policy" across configuration files and the Java client README. The review feedback correctly identifies a couple of instances in the README where the name update resulted in a duplicate "Google" prefix (e.g., "Google Google Cloud"), and provides suggestions to clean those up.

[zhumin8]

@jinseopkim0 is the name change related to the fix in the script fix in #13378, or was it only cosmetic? After the templated README generation, it now creates this "Google Google Cloud IAM Policy Client for Java" name. Can we remove "Google" and use "Cloud IAM Policy" instead?

[zhumin8]

per chat discussion, updated name.