Avoid datetime deprecations in releaseWizard.py (#751)

Python 3.12 deprecates two popular datetime functions that are used
throughout the Release Wizard, 'utcnow()' and 'utcfromtimestamp()',
resulting in many distracting messages that make the other wizard output
hard to read, e.g.

```
DeprecationWarning: datetime.datetime.utcnow() is deprecated and
scheduled for removal in a future version. Use timezone-aware objects to
represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
```

This commit updates our 'datetime' usage in accordance with the Python
recommendations.

hack/release/wizard/releaseWizard.py

@@ -49,6 +49,7 @@
 from collections import OrderedDict
 from datetime import datetime
 from datetime import timedelta
+from datetime import timezone
 
 try:
     import holidays
@@ -97,7 +98,7 @@ def expand_jinja(text, vars=None):
         'state': state,
         'gpg_key': state.get_gpg_key(),
         'gpg_fingerprint': state.get_gpg_fingerprint(),
-        'epoch': unix_time_millis(datetime.utcnow()),
+        'epoch': unix_time_millis(datetime.now(timezone.utc)),
         'get_next_version': state.get_next_version(),
         'get_next_major_version': state.get_next_major_version(),
         'get_next_minor_version': state.get_next_minor_version(),
@@ -201,7 +202,7 @@ def check_prerequisites(todo=None):
     return True
 
 
-epoch = datetime.utcfromtimestamp(0)
+epoch = datetime.fromtimestamp(0, timezone.utc)
 
 
 def unix_time_millis(dt):
@@ -295,7 +296,7 @@ def __init__(self, config_path, release_version, script_version):
         self.latest_version = None
         self.previous_rcs = {}
         self.rc_number = 1
-        self.start_date = unix_time_millis(datetime.utcnow())
+        self.start_date = unix_time_millis(datetime.now(timezone.utc))
         self.script_branch = run("git rev-parse --abbrev-ref HEAD").strip()
         self.released_versions = None
         try:
@@ -771,7 +772,7 @@ def get_vars(self):
 
     def set_done(self, is_done):
         if is_done:
-            self.state['done_date'] = unix_time_millis(datetime.utcnow())
+            self.state['done_date'] = unix_time_millis(datetime.now(timezone.utc))
             if self.persist_vars:
                 for k in self.persist_vars:
                     self.state[k] = self.get_vars()[k]
@@ -966,21 +967,21 @@ def expand_multiline(cmd_txt, indent=0):
 
 
 def unix_to_datetime(unix_stamp):
-    return datetime.utcfromtimestamp(unix_stamp / 1000)
+    return datetime.fromtimestamp(unix_stamp / 1000, timezone.utc)
 
 
 def generate_asciidoc():
     base_filename = os.path.join(state.get_release_folder(),
                                  "solr_operator_release_%s"
-                                 % (state.release_version.replace("\.", "_")))
+                                 % (state.release_version.replace(".", "_")))
 
     filename_adoc = "%s.adoc" % base_filename
     filename_html = "%s.html" % base_filename
     fh = open(filename_adoc, "w")
 
     fh.write("= Solr Operator Release %s\n\n" % state.release_version)
     fh.write("(_Generated by releaseWizard.py %s at %s_)\n\n"
-             % (getScriptVersion(), datetime.utcnow().strftime("%Y-%m-%d %H:%M UTC")))
+             % (getScriptVersion(), datetime.now(timezone.utc).strftime("%Y-%m-%d %H:%M UTC")))
     fh.write(":numbered:\n\n")
     fh.write("%s\n\n" % template('help'))
     for group in state.todo_groups:
@@ -1895,17 +1896,17 @@ def create_ical(todo):
     return True
 
 
-today = datetime.utcnow().date()
+today = datetime.now(timezone.utc).date()
 sundays = {(today + timedelta(days=x)): 'Sunday' for x in range(10) if (today + timedelta(days=x)).weekday() == 6}
-y = datetime.utcnow().year
+y = datetime.now(timezone.utc).year
 years = [y, y+1]
 non_working = holidays.CA(years=years) + holidays.US(years=years) + holidays.UK(years=years) \
               + holidays.DE(years=years) + holidays.NO(years=years) + holidays.IND(years=years) + holidays.RU(years=years)
 
 
 def vote_close_72h_date():
     # Voting open at least 72 hours according to ASF policy
-    return datetime.utcnow() + timedelta(hours=73)
+    return datetime.now(timezone.utc) + timedelta(hours=73)
 
 
 def vote_close_72h_holidays():