fix(model): handle true timestamp fields correctly (#12580)

[papb]

This is a cherry-pick from #12580 targeting the v6 branch, so that the fix can land in v6.

[codecov]

Codecov Report

Merging #12581 into v6 will increase coverage by 0.40%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##               v6   #12581      +/-   ##
==========================================
+ Coverage   95.97%   96.38%   +0.40%     
==========================================
  Files          92       95       +3     
  Lines        8953     9162     +209     
==========================================
+ Hits         8593     8831     +238     
+ Misses        360      331      -29     

Impacted Files	Coverage Δ
lib/model.js	96.58% <87.50%> (-0.06%)	⬇️
lib/dialects/postgres/connection-manager.js	92.30% <0.00%> (-3.50%)	⬇️
lib/dialects/mariadb/index.js	100.00% <0.00%> (ø)
lib/dialects/mariadb/connection-manager.js	100.00% <0.00%> (ø)
lib/dialects/mariadb/query.js	87.96% <0.00%> (ø)
lib/sequelize.js	95.87% <0.00%> (+0.63%)	⬆️
lib/dialects/abstract/query-interface.js	94.08% <0.00%> (+1.86%)	⬆️
lib/dialects/mariadb/data-types.js	100.00% <0.00%> (+52.63%)	⬆️
lib/dialects/mariadb/query-generator.js	100.00% <0.00%> (+81.25%)	⬆️

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[sushantdhiman]

🎉 This PR is included in version 6.3.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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