Support db.G[T]() with Go 1.27 generic methods

Describe the feature

Now that Go 1.27 supports generic methods, expose GORM's generic API directly from *gorm.DB:

db.G[User]()

(*gorm.DB).G[T] should delegate to the existing gorm.G[T](db, opts...) implementation so both forms have identical behavior and options.

For backwards compatibility:

• Compile the method only for Go 1.27 and newer using a version build constraint.
• Keep gorm.G[T](db) available and unchanged.
• Do not raise GORM's minimum supported Go version.
• Test the receiver method with Go 1.27+ and continue testing the package-level API on older supported Go versions.

Motivation

Before Go 1.27, the language did not permit generic methods, so GORM had to expose this API as a package-level function:

gorm.G[User](db)

The receiver-based form is easier to discover, reads naturally in query chains, and avoids repeating both the package name and database argument:

users, err := db.G[User]().
    Where("active = ?", true).
    Find(ctx)

Related Issues

Implementation: #7849