Evaluate minimum pip version and native pip freeze exclusions #916

Consider raising the minimum supported pip version and using pip freeze --exclude rsconnect-python --exclude rsconnect for dependency inspection.

In the review of #909, @amol- suggested using pip's native exclusion flags to simplify filtering. Those flags are unavailable in pip 20. Changing the supported pip versions deserves a separate compatibility decision from the requirement-filtering fix in #899.

Source: #909 (comment)
Original proposal: #909 (comment)

Scope

• Determine the minimum pip version that supports the proposed flags and assess the impact on supported Python and deployment environments.
• Decide whether to raise the minimum version, retain a compatibility fallback, or keep the current approach. Document the compatibility decision.
• If adopted, use the exclusion flags when inspecting dependencies with pip and update compatibility checks and documentation as needed.
• Preserve the shared requirement filter for user-provided requirements files and other dependency sources; pip flags do not cover those paths or replace local editable-install validation.

Validation

• Exercise dependency inspection at the chosen minimum pip version and a current version.
• Verify that rsconnect distributions are excluded while unrelated packages and paths containing rsconnect are preserved.
• Verify the selected behavior for older pip versions, including a clear error if support is dropped.
• Keep coverage for requirements-file filtering and local editable dependencies.

Related: #899, #857, #909.