Resolve urllib3 conflict and update to v2.6.3

- Remove appengine-python-standard which is no longer needed with
  modern dependencies and restricted urllib3 to <2.
- Update urllib3 to >=2.5.0 in pyproject.toml to address CVEs.
- Update urllib3 to 2.6.3 in test requirements.

Fixes #3325

Signed-off-by: Luciano Resende <lresende@apple.com>

Author: lresende

elyra/tests/kfp/resources/test-requirements-elyra.txt

@@ -17,5 +17,5 @@ pyzmq==26.4.0
 requests==2.25.1
 tornado==6.1.0
 traitlets==4.3.3
-urllib3==1.26.5
+urllib3==2.6.3
 zebra===0.1.32

pyproject.toml

@@ -46,12 +46,10 @@ dependencies = [
     "tornado>=6.1.0",
     "traitlets>=4.3.2",
     "typing-extensions>=3.10",
-    "urllib3>=1.26.5",
+    "urllib3>=2.5.0",
     "watchdog>=2.1.3",
     "websocket-client",
     "yaspin",
-    # see: https://stackoverflow.com/questions/76175487/sudden-importerror-cannot-import-name-appengine-from-requests-packages-urlli
-    "appengine-python-standard",
     "kfp>=2.0.0", # Cap kfp for protobuf compatibility
     "kfp-kubernetes>=1.0.0",
     "pygithub",