chore(deps): bump tracing-subscriber from 0.3.14 to 0.3.15 in /backend (#248)

Bumps [tracing-subscriber](https://github.com/tokio-rs/tracing) from 0.3.14 to 0.3.15.
- [Release notes](https://github.com/tokio-rs/tracing/releases)
- [Commits](https://github.com/tokio-rs/tracing/compare/tracing-subscriber-0.3.14...tracing-subscriber-0.3.15)

---
updated-dependencies:
- dependency-name: tracing-subscriber
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

backend/Cargo.lock

@@ -3995,9 +3995,9 @@ dependencies = [
 
 [[package]]
 name = "tracing-subscriber"
-version = "0.3.14"
+version = "0.3.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3a713421342a5a666b7577783721d3117f1b69a393df803ee17bb73b1e122a59"
+checksum = "60db860322da191b40952ad9affe65ea23e7dd6a5c442c2c42865810c6ab8e6b"
 dependencies = [
  "ansi_term",
  "matchers",