chore(tvix/tracing): bump to opentelemetry 0.28

According to https://github.com/open-telemetry/opentelemetry-rust/issues/1395#issuecomment-2612865575,
we can now remove the spawn_blocking calls and channel logic for
flushing / shutdown.

Change-Id: I0fabc1fe0968a736e7d44dfde41f4a6adac5e272
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13154
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

tvix/store/src/bin/tvix-store.rs

@@ -541,7 +541,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     tokio::select! {
         res = tokio::signal::ctrl_c() => {
             res?;
-            if let Err(e) = tracing_handle.force_shutdown().await {
+            if let Err(e) = tracing_handle.shutdown().await {
                 eprintln!("failed to shutdown tracing: {e}");
             }
             Ok(())