CI: concurrency=complete require-explicit-sendable

Motivation:

Adopt `-Xswiftc -strict-concurrency=complete -Xswiftc
-require-explicit-sendable` in CI to lock-in concurrency-safety wins.

Modifications:

Added strict flags to pull request and scheduled GHA CI on Swift 6.0.

Result:

CI should ensure we don't regress data-race safety on Swift 6.0.

.github/workflows/pull_request.yml

@@ -16,8 +16,9 @@ jobs:
  with:
  linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
  linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
- linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
- linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+ linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"
+ linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"
+ linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"
 
  benchmarks:
  name: Benchmarks

.github/workflows/scheduled.yml

@@ -12,5 +12,6 @@ jobs:
  linux_5_8_enabled: false
  linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
  linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
- linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
- linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+ linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"
+ linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"
+ linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete -Xswiftc -require-explicit-sendable"