Make tracing Optional for bevy_utils

[bushrat011899]

Objective

• Allows bevy_utils to compile on platforms without atomic CAS operations (e.g., GameBoy Advance thumbv4t-none-eabi)
• Contributes to Tracking Issue: MVP no_std Bevy #15460

Solution

• Made tracing optional (behind the tracing feature, enabled by default).
• Switched to using log within bevy_utils, and exported it as log.

Testing

• cargo build --target thumbv4t-none-eabi

Migration Guide

If you were using bevy_utils with default features disabled, and you relied on the tracing re-export, either enable the tracing feature, or switch to using the log export, or import tracing into your project directly.

Notes

tracing has built-in support for capturing logs from log and this is enabled by default. The biggest loss going from log to tracing is the loss of info_span/etc. Otherwise, the crates are largely identical in behaviour, with log having no dependencies and more popularity on crates.io.