Prepare for v0.20.0 release (#41)

Changelog:

### Added

- Add `OpenTelemetrySpanExt::set_attribute` function (#34)

### Breaking Changes

- Upgrade to `v0.20.0` of `opentelemetry` (#36)
  For list of breaking changes in OpenTelemetry, see the
[v0.20.0
changelog](https://github.com/open-telemetry/opentelemetry-rust/blob/v0.20.0/opentelemetry-api/CHANGELOG.md#v0200).

Author: jtescher

CHANGELOG.md

@@ -1,3 +1,17 @@
+# 0.20.0 (August 1, 2023)
+
+### Added
+
+- Add `OpenTelemetrySpanExt::set_attribute` function (#34)
+
+### Breaking Changes
+
+- Upgrade to `v0.20.0` of `opentelemetry` (#36)
+  For list of breaking changes in OpenTelemetry, see the
+  [v0.20.0 changelog](https://github.com/open-telemetry/opentelemetry-rust/blob/v0.20.0/opentelemetry-api/CHANGELOG.md#v0200).
+
+Thanks to @ymgyt, @mladedav, @shaun-cox, and @Protryon for contributing to this release!
+
 # 0.19.0 (May 23, 2023)
 
 ### Breaking Changes

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tracing-opentelemetry"
-version = "0.19.0"
+version = "0.20.0"
 authors = [
     "Julian Tescher <[email protected]>",
     "Tokio Contributors <[email protected]>"