Rename to ibet-Core

[YoshihitoAso]

This pull request rebrands the project from "Quorum" or "GoQuorum for ibet Network" to "ibet-Core" and updates documentation and configuration to clarify the project's relationship to GoQuorum and go-ethereum. It removes outdated or redundant documentation and improves clarity around the project's features, build instructions, and licensing.

Project rebranding and documentation updates:

• Updated the project name from "GoQuorum for ibet Network" to "ibet-Core" throughout README.md, and clarified that ibet-Core is a GoQuorum-derived client optimized for ibet Network. The overview, features, and version control policy sections were rewritten for clarity.
• Expanded and clarified the license section in README.md to explain the relationship to upstream GoQuorum and go-ethereum code, and to specify licensing for different parts of the codebase.

Documentation cleanup:

• Removed the outdated BUILDING.md file in favor of updated build instructions now present in README.md.
• Removed the old NOTES.md file, which contained historical notes about Quorum's private state implementation that are no longer relevant to ibet-Core.

Configuration update:

• Updated the comment in .github/dependabot.yml to reference "ibet-Core" instead of "Quorum" for dependency pinning rationale.