emulators/flycast: Update 2.4 => 2.5

Changelog:
https://github.com/flyinghead/flycast/releases/tag/v2.5

• Add net/asio to BUILD_DEPENDS.
• Update to USES=compiler:c++20-lang.
• Remove upstreamed patches.
• Cosmetic non-functional and whitespace changes.

PR: 286653