tor-error, arti: Bump backtrace to 0.3.68.

Previously we were using backtrace 0.3.39, which has a [bug] that causes
it to segault in some circumstances. I experienced this bug while trying
to fix the minimal-versions build in !1508.

[bug]: https://github.com/rust-lang/backtrace-rs/issues/267

crates/arti/Cargo.toml

@@ -91,7 +91,7 @@ arti-rpcserver = { path = "../arti-rpcserver", version = "0.2.0", optional = tru
 anyhow = "1.0.23"
 arti-client = { package = "arti-client", path = "../arti-client", version = "0.10.0", default-features = false }
 async-ctrlc = { version = "1.2.0", optional = true }
-backtrace = "0.3.39"
+backtrace = "0.3.68"
 cfg-if = "1.0.0"
 clap = "3.2.20"
 config = { version = "0.13", default-features = false, features = ["toml"] }

crates/tor-error/Cargo.toml

@@ -25,7 +25,7 @@ tracing = ["dep:tracing", "static_assertions"]
 __is_experimental = []
 
 [dependencies]
-backtrace = { version = "0.3.39", optional = true }
+backtrace = { version = "0.3.68", optional = true }
 derive_more = "0.99.3"
 futures = "0.3"
 once_cell = "1"