FIX: Handle .discourse-compatibility syntax errors (#10891)

Previously, any errors in those files would e.g. blow up the update process in docker_manager.
Now it prints out an error and proceeds as if there was no compatibility file.

Includes:

* DEV: Extract setup_git_repo
* DEV: Use `Dir.mktmpdir`
* DEV: Default to `main` branch (The latest versions of git already do this, so to avoid problems do this by default)

lib/version.rb

@@ -16,6 +16,8 @@ module Discourse
     end
   end
 
+  class InvalidVersionListError < StandardError; end
+
   def self.has_needed_version?(current, needed)
     Gem::Version.new(current) >= Gem::Version.new(needed)
   end
@@ -29,10 +31,16 @@ module Discourse
   #  2.4.4.beta6: some-other-branch-ref
   #  2.4.2.beta1: v1-tag
   def self.find_compatible_resource(version_list, version = ::Discourse::VERSION::STRING)
-
     return unless version_list
 
-    version_list = YAML.load(version_list).sort_by { |v, pin| Gem::Version.new(v) }.reverse
+    begin
+      version_list = YAML.safe_load(version_list)
+    rescue Psych::SyntaxError, Psych::DisallowedClass => e
+    end
+
+    raise InvalidVersionListError unless version_list.is_a?(Hash)
+
+    version_list = version_list.sort_by { |v, pin| Gem::Version.new(v) }.reverse
 
     # If plugin compat version is listed as less than current Discourse version, take the version/hash listed before.
     checkout_version = nil
@@ -54,5 +62,7 @@ module Discourse
     return unless File.directory?("#{path}/.git")
     compat_resource, std_error, s = Open3.capture3("git -C '#{path}' show HEAD@{upstream}:#{Discourse::VERSION_COMPATIBILITY_FILENAME}")
     Discourse.find_compatible_resource(compat_resource) if s.success?
+  rescue InvalidVersionListError => e
+    $stderr.puts "Invalid version list in #{path}"
   end
 end