Use ubuntu 20.04 for github runners instead of 18.04

Author: LBPHacker

.github/prepare.py

@@ -19,7 +19,7 @@ def set_output(key, value):
 
 configurations = []
 for bsh_host_arch, bsh_host_platform, bsh_host_libc, bsh_static_dynamic, bsh_build_platform,        runs_on in [
-	(   'x86_64' ,           'linux',         'gnu',           'static',            'linux', 'ubuntu-18.04' ),
+	(   'x86_64' ,           'linux',         'gnu',           'static',            'linux', 'ubuntu-20.04' ),
 	(   'x86_64' ,         'windows',       'mingw',           'static',          'windows', 'windows-2019' ),
 	(   'x86_64' ,         'windows',       'mingw',          'dynamic',          'windows', 'windows-2019' ),
 	(   'x86_64' ,         'windows',        'msvc',           'static',          'windows', 'windows-2019' ),