Remove packaging as a runtime dependency of opentelemetry-instrumentation-flask

[ocelotl]

Description

Removes packaging as a runtime dependency of opentelemetry-instrumentation-flask.
The flask.__version__ comparison now uses the internal PEP 440 Version from
opentelemetry.instrumentation._packaging.version (provided by
opentelemetry-instrumentation) instead of packaging, and packaging is
dropped from this package's dependencies.

Fixes #4885

Depends on #4882 (draft #4883), which adds
opentelemetry.instrumentation._packaging to opentelemetry-instrumentation.
This PR's CI will not pass until that lands; kept as a draft until then.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Behavior is unchanged; the version-comparison call sites are identical apart from
the import source. Covered by the existing test suite for this instrumentation
once #4882 is merged and this branch is rebased.

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated