Replace md5 hashes with sha256 (data backend side has already been implemented see: https://mapgen-data.xcsoar.org/)

Codacy detected an issue:

Message: Use of insecure MD2, MD4, MD5, or SHA1 hash function.

Currently on:

• Commit: 12275f7
• File: lib/xcsoar/mapgen/downloader.py
• LineNum: 99