fix: remove protocol definition default args deprecated in Elixir 1.20

[rafaels88]

Default arguments in defprotocol definitions are deprecated in Elixir 1.20 and will become a hard error in a future release.

• Remove \ :microsecond defaults from Timex.Comparable.compare/3 and diff/3
• Remove \ :utc default from Timex.Protocol.to_datetime/2
• Drop now-invalid 1-arity @spec for to_datetime
• Change defdelegate compare/2 and diff/2 in Timex to explicit calls with hardcoded :microsecond default to preserve public API

The warnings:

warning: default arguments in protocol definitions is deprecated
│
84 │   def compare(a, b, granularity \\ :microsecond)
│   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
│
└─ lib/comparable/comparable.ex:84: Timex.Comparable (module)

 warning: default arguments in protocol definitions is deprecated
 │
139 │   def diff(a, b, granularity \\ :microsecond)
│   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
│
└─ lib/comparable/comparable.ex:139: Timex.Comparable (module)

warning: default arguments in protocol definitions is deprecated
│
48 │   def to_datetime(datetime, timezone \\ :utc)
│   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
│
└─ lib/protocol.ex:48: Timex.Protocol (module)

Note there's also a secondary warning that comes with each ofault silently creates a lower-arity function that protocolsaren't supposed to have:

warning: protocols can only define functions without implementation via def/1, found: compare/2, diff/2
│
 1 │ defprotocol Timex.Comparable do
│   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
│
└─ lib/comparable/comparable.ex:1: Timex.Comparable (module)

Summary of changes

Elixir 1.20 deprecated default arguments inside defprotocol definitions — they produce both f/n and f/n-1 as protocol variants, which will become a hard compile error in a future version. Timex 3.7.13 has three such defaults: compare/3, diff/3 in Timex.Comparable, and to_datetime/2 in Timex.Protocol.

Projects running --warnings-as-errors (a common CI setup) can't compile against this on Elixir 1.20.

This removes the three defaults from the protocol definitions and makes the 2-arity public wrappers in Timex supply the hardcoded defaults explicitly, so callers see no behaviour change.

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes - it's not testable
• Commits were squashed into a single coherent commit
• Notes added to CHANGELOG file which describe changes at a high-level