Rename `is_GlobalGenus`, `Genus_Symbol_p_adic_ring`

[mkoeppe]

Names in the sage.quadratic_forms.genera.genus module should be reworked for uniformity:

• the global function is_GlobalGenus should be renamed so it avoids the deprecated pattern of is_Class functions (Meta-ticket: Replace is_Class functions, create abstract base classes to enable modularization #32414) or maybe become a method of GenusSymbol_global_ring
• Genus_Symbol_p_adic_ring should probably be renamed to be in line with other classes in the same module that follow the normal naming scheme, such as GenusSymbol_global_ring