Revert default of allow_vendor_change to true

We discovered several issues related to the feature.

Command line packages are refused to get installed when they have
a different vendor.

Disabling copr repository results in refusing installation of packages
and upgrades from original repositories - skipping all upgrades.

Cryptic solver message
 - it is impossible to guess that the real issue is related to
 a different vendor of both packages
 - cannot install both dnf5-5.0.13-2.fc38.x86_64 and dnf5-5.1.0-20230719020203.0.gc17c4f61.fc38.x86_64
    repoquery info does not show vendor

libdnf5/conf/config_main.cpp

@@ -210,7 +210,7 @@ class ConfigMain::Impl {
  OptionBool gpgkey_dns_verification{false};
  OptionBool obsoletes{true};
  OptionBool exit_on_lock{false};
- OptionBool allow_vendor_change{false};
+ OptionBool allow_vendor_change{true};
  OptionSeconds metadata_timer_sync{60 * 60 * 3}; // 3 hours
  OptionStringList disable_excludes{std::vector<std::string>{}};
  OptionEnum<std::string> multilib_policy{"best", {"best", "all"}}; // :api