Fix Elixir 1.20 warnings

[nyo16]

• Add pin operator (^) to variables used inside size(...) in bitstring patterns, as required by Elixir 1.20
• Fixes 22 compilation warnings introduced by the new Elixir 1.20 requirement that variables defined outside a match must be pinned when used in size(...)
• Changes in Protobuf.Decoder (3 locations) and Protobuf.JSON.RFC3339 (2 locations)

Test plan

• mix compile --force --all-warnings produces 0 warnings
• mix test — all 483 tests pass with no failures"2>&1

[whatyouhide]

@nyo16 this breaks compatibility with older versions. We can bump version req to 1.15 but we'll need to address those if they are problematic there as well. Could you take a look?

[nyo16]

Thanks for the review! I dug into this — the ^ pin operator in binary size specifiers was introduced in Elixir 1.16.0 (verified against the compiler source), so bumping to 1.15 alone
wouldn't cover it.

That said, these are just warnings on Elixir 1.20, not errors — the code without ^ still compiles and works fine. So if maintaining broader compatibility is more important, it's totally
reasonable to leave things as-is for now. I can keep this on my fork for my own sanity (OCD) in the meantime.

If you do want to address them, I've updated the PR to:

• Bump minimum Elixir to ~> 1.16 in mix.exs
• Update CI matrix and conformance tests to match (1.16.3/OTP 26.0)

Happy to go either way — let me know what you prefer.