Deprecating PostgresDataConvertible, PostgresMessageType

[fabianfett]

Motivation

We want to inform users of patterns that will be going away.

Changes

• deprecate PostgresDataConvertible
• deprecate PostgresMessageType
• deprecate PostgresJSONCodable
• deprecate PostgresJSONBCodable

[gwynne]

Deprecating PostgresDataConvertible would cause postgres-kit to emit multiple deprecation warnings in anything using it (typically via Fluent) which users would have no way to silence. @0xTim any thoughts?

[codecov-commenter]

Codecov Report

Merging #313 (6eb244e) into main (606c68a) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 6eb244e differs from pull request most recent head eac6716. Consider uploading reports for the commit eac6716 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #313   +/-   ##
=======================================
  Coverage   41.27%   41.27%           
=======================================
  Files         115      115           
  Lines        9572     9572           
=======================================
  Hits         3951     3951           
  Misses       5621     5621           

Impacted Files	Coverage Δ
Sources/PostgresNIO/Data/PostgresData+Array.swift	0.00% <0.00%> (ø)
Sources/PostgresNIO/Data/PostgresData+Bool.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+Bytes.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+Date.swift	0.00% <ø> (ø)
...ources/PostgresNIO/Data/PostgresData+Decimal.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+Double.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+Float.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+Int.swift	0.00% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+JSON.swift	61.90% <ø> (ø)
Sources/PostgresNIO/Data/PostgresData+JSONB.swift	88.23% <ø> (ø)
... and 16 more

... and 2 files with indirect coverage changes

[gwynne]

This PR will be unblocked by completion of vapor/postgres-kit#237, at that time there are no further objections to merging it.