chore: Focus on actual errors by removing -warnings-as-errors

[dongjoon-hyun]

What's Changed

This PR aims to focus on real errors by removing -warnings-as-errors flag.

To make it sure, this PR removes the following from CI test pipelines.

arrow-swift/.github/workflows/test.yaml

Lines 89 to 90 in b6ca546

	# We remove this when we support Swift 6.0 or later.
	continue-on-error: ${{ matrix.swift-version != '5.10' }}

We can enable -warnings-as-errors later .

[dongjoon-hyun]

cc @kou

[dongjoon-hyun]

All Swift versions (5.10, 6.0, 6.1) passed.

Instead of focusing on warnings, we need to focus other stuffs first. For example,

• Upgrading GRPC Swift to GRPC Swift 2. It's because some libraries are officially tested with Swift 6.1 at the latest versions.
• Adding Decimal type, GH-46627: [Swift] Support Decimal128 arrow#46628 .

If you don't mind, why don't we call it a day for Swift 6.x support with this PR, @kou ?

[kou]

Hmm. I don't want to remove this as much as possible to keep healthy code.

[dongjoon-hyun]

Got it.