Release 7.3.0

Support for Elasticsearch v7.3.

• Added support for simplified access to the X-Opaque-Id header #952
• Added the HTTP port in the log messages #950
• Fixed hostname with underscore (ClientBuilder::prependMissingScheme) #949
• Removed unused Monolog classes in ClientBuilder #948

Note: this release is also compatible with Elasticsearch v7.2.