Replace VERSION.TXT parsing with importlib.metadata for Drake version detection

[Copilot]

GetDrakeVersion() and DrakeVersionGreaterThan() relied on reading a VERSION.TXT file from disk, which only exists for certain Drake install types and breaks for others (pip, rules_python, source builds, etc.).

Changes

• GetDrakeVersion(): Replaced VERSION.TXT file parsing with importlib.metadata.version("drake"). Returns "source" on PackageNotFoundError (e.g. unregistered source installs).
• DrakeVersionGreaterThan(): Now delegates to GetDrakeVersion() instead of re-reading the file. Updated nightly version parsing to handle the "0.0.YYYYMMDD" format returned by importlib.metadata (previously parsed a "YYYYMMDDHHMMSS" string from VERSION.TXT).
• Removed unused from pathlib import Path import.

import importlib.metadata
importlib.metadata.version("drake")  # "1.52.1" or "0.0.20260303" for nightlies

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[RussTedrake]

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[RussTedrake]

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