[web] Stop using negative indices with os.stat results The python doc says: More items may be added at the end by some implementations. .st_mtime is also more readable than [-2].

Release notes
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.. toctree::
    :maxdepth: 1

    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14