deprecate time datatype

[dpinol]

time datatype is deprecated and broken according to https://docs.h5py.org/en/stable/special.html#storing-other-types-as-opaque-data

Closes #465

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (eb6b2ae) 84.04% compared to head (ba71e01) 84.04%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #467 +/- ## ======================================= Coverage 84.04% 84.04% ======================================= Files 31 31 Lines 4142 4142 ======================================= Hits 3481 3481 Misses 661 661 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/467?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/datatypes.jl](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/467?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2RhdGF0eXBlcy5qbA==) | `86.59% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[JonasIsensee]

Thank you!