Update der requirement from 0.6.1 to 0.7.8

Updates the requirements on [der](https://github.com/RustCrypto/formats) to permit the latest version.
- [Commits](RustCrypto/formats@der/v0.6.1...der/v0.7.8)

---
updated-dependencies:
- dependency-name: der
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -20,7 +20,7 @@ name = "hifitime"
 [dependencies]
 serde = {version = "1.0.155", optional = true}
 serde_derive = {version = "1.0.155", optional = true}
-der = {version = "0.6.1", features = ["derive", "real"], optional = true}
+der = {version = "0.7.8", features = ["derive", "real"], optional = true}
 pyo3 = { version = "0.19.0", features = ["extension-module"], optional = true }
 num-traits = {version = "0.2.15", default-features = false, features = ["libm"]}
 lexical-core = {version = "0.8.5", default-features = false, features = ["parse-integers", "parse-floats"]}