api: Turn on RFC 3986 parsing by default and update javadoc.

[jdcormie]

Removed the "NameResolver-compliant" terminology because it's no longer
needed. All RFC 3986 URIs are absolute in the sense that have a scheme
(URI != "URI-reference"). RFC 3986 also does away with the hierarchical
vs opaque distinction. All URIs now have a hierarchy of components
(scheme, authority, path, query and fragment) it's just that some components
can be absent and/or the empty string.

Added discussion of ambiguity which is technically new with the RFC 3986
change. If you believed the current javadoc, a target like foo:8888
didn't used to be ambiguous because it is not a "valid NameResolver-
compliant" URI -- the path doesn't start with / and so java.net.URI
considers it opaque, not hierarchical as required. Fortunately (???)
ManagedChannelImplBuilder never enforced the !isOpaque() requirement
mentioned in the javadoc and so this ambiguity isn't actually new in
practice. gRPC C++ also appears to suffer from the same problem.

Will merge in sync with CL/888346527.

[Copilot]

Pull request overview

This PR enables RFC 3986 target parsing by default and updates public API Javadoc to describe RFC 3986 URIs (instead of the legacy java.net.URI hierarchical/opaque behavior), including new guidance about ambiguity when using authority-string targets.

Changes:

• Flip GRPC_ENABLE_RFC3986_URIS default to true.
• Update ManagedChannelBuilder.forTarget() and Grpc.newChannelBuilder() Javadoc to describe RFC 3986 URI targets and authority-string conversion.
• Add Javadoc discussion and examples highlighting ambiguity for targets like foo:8080.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
api/src/main/java/io/grpc/ManagedChannelBuilder.java	Updates forTarget() Javadoc for RFC 3986 parsing and ambiguity guidance.
api/src/main/java/io/grpc/Grpc.java	Updates newChannelBuilder() Javadoc to match RFC 3986 parsing and ambiguity guidance.
api/src/main/java/io/grpc/FeatureFlags.java	Turns RFC 3986 parsing on by default via feature flag default change.

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