"Downgrade" warning when regenerating after version bump

There is nothing to "warn" about, this is a completely routine
occurence. OTOH, when something is corrupted, we should warn. Keep
the red color and "WARNING:" prefix in that case.

Example output:
$ ninja -C build
Regenerating configuration from scratch: Build directory has been generated with Meson version 0.55.999, which is incompatible with current version 0.56.0.
The Meson build system
Version: 0.56.0
...

3 files changed, 18 insertions, 5 deletions

diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index cf418bb..ce03fbc 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -45,6 +45,16 @@ default_yielding = False
 # Can't bind this near the class method it seems, sadly.
 _T = T.TypeVar('_T')
 
+class MesonVersionMismatchException(MesonException):
+    '''Build directory generated with Meson version incompatible with current version'''
+    def __init__(self, old_version, current_version):
+        super().__init__('Build directory has been generated with Meson version {}, '
+                         'which is incompatible with current version {}.'
+                         .format(old_version, current_version))
+        self.old_version = old_version
+        self.current_version = current_version
+
+
 class UserOption(T.Generic[_T]):
     def __init__(self, description, choices, yielding):
         super().__init__()
@@ -982,9 +992,7 @@ def load(build_dir):
     if not isinstance(obj, CoreData):
         raise MesonException(load_fail_msg)
     if major_versions_differ(obj.version, version):
-        raise MesonException('Build directory has been generated with Meson version %s, '
-                             'which is incompatible with current version %s.\n' %
-                             (obj.version, version))
+        raise MesonVersionMismatchException(obj.version, version)
     return obj
 
 def save(obj, build_dir):
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 01fc8e4..6987863 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -523,6 +523,11 @@ class Environment:
                 self.first_invocation = False
             except FileNotFoundError:
                 self.create_new_coredata(options)
+            except coredata.MesonVersionMismatchException as e:
+                # This is routine, but tell the user the update happened
+                mlog.log('Regenerating configuration from scratch:', str(e))
+                coredata.read_cmd_line_file(self.build_dir, options)
+                self.create_new_coredata(options)
             except MesonException as e:
                 # If we stored previous command line options, we can recover from
                 # a broken/outdated coredata.
diff --git a/run_unittests.py b/run_unittests.py
index 0418ccf..49e2206 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -4030,7 +4030,7 @@ recommended as it is not supported on some platforms''')
         self.__reconfigure()
 
         out = self.init(testdir, extra_args=['--reconfigure', '-Dopt3=val3'])
-        self.assertRegex(out, 'WARNING:.*Regenerating configuration from scratch')
+        self.assertRegex(out, 'Regenerating configuration from scratch')
         self.assertRegex(out, 'opt1 val1')
         self.assertRegex(out, 'opt2 val2')
         self.assertRegex(out, 'opt3 val3')
@@ -4067,7 +4067,7 @@ recommended as it is not supported on some platforms''')
         self.__reconfigure(change_minor=True)
 
         out = self.init(testdir, extra_args=['--reconfigure', '-Dopt3=val3'])
-        self.assertNotRegex(out, 'WARNING:.*Regenerating configuration from scratch')
+        self.assertNotRegex(out, 'Regenerating configuration from scratch')
         self.assertRegex(out, 'opt1 val1')
         self.assertRegex(out, 'opt2 val2')
         self.assertRegex(out, 'opt3 val3')