upgrade Pex to 2.3.0 (#20734)

Changelog: https://github.com/pex-tool/pex/releases/tag/v2.3.0

 - `sync` of interest for #15704
 - error message clarification regarding #15062
 - fix for explicit flags as implemented #20598

```
Lockfile diff: 3rdparty/python/user_reqs.lock [python-default]

==                    Upgraded dependencies                     ==

  asgiref                        3.7.2        -->   3.8.1
  cryptography                   42.0.3       -->   42.0.5
  pex                            2.2.1        -->   2.3.0
  pyparsing                      3.1.1        -->   3.1.2
  python-dateutil                2.8.2        -->   2.9.0.post0
  sniffio                        1.3.0        -->   1.3.1
```

3rdparty/python/requirements.txt

@@ -10,7 +10,7 @@ fasteners==0.16.3
 freezegun==1.2.1
 ijson==3.1.4
 packaging==21.3
-pex==2.2.1
+pex==2.3.0
 psutil==5.9.0
 # This should be compatible with pytest.py, although it can be looser so that we don't
 # over-constrain pantsbuild.pants.testutil