Release 1.6.0

Breaking changes

• Requires PHP 8.2, and no longer requires symfony/polyfill-php82 (#2091).

Changed

• EnvironmentGetterSetter normalizes keys to environment variable names. Letters are
  uppercased, any other character outside A-Z, 0-9 and _ becomes _, and a leading digit
  gets a _ prefix. An empty key is written as _, where it used to throw (#2092).
• EnvironmentGetterSetter::keys() returns the names of variables that are already normalized, as
  they are. It used to return every variable name, lowercased (#2092).

Full Changelog: 1.5.0...1.6.0