meson: Increase minimum version to 0.57.2

 meson: Increase minimum version to 0.57.2
 
 The previous minimum was to maintain support for Python 3.5, but we
 now require Python 3.6 anyway (commit 45363fca637), so that reason is
 obsolete.  A small raise to Meson 0.57 allows getting rid of a fair
 amount of version conditionals and silences some future-deprecated
 warnings.
 
 With the version bump, the following deprecation warnings appeared and
 are fixed:
 
 WARNING: Project targets '>=0.57' but uses feature deprecated since '0.55.0': ExternalProgram.path. use ExternalProgram.full_path() instead
 WARNING: Project targets '>=0.57' but uses feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.
 
 It turns out that meson 0.57.0 and 0.57.1 are buggy for our use, so
 the minimum is actually set to 0.57.2.  This is specific to this
 version series; in the future we won't necessarily need to be this
 precise.
 
 Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
 Reviewed-by: Andres Freund <andres@anarazel.de>
 Discussion: https://www.postgresql.org/message-id/flat/42e13eb0-862a-441e-8d84-4f0fd5f6def0%40eisentraut.org