Update opentelemetry-exporter-otlp requirement from ~> 0.28.1 to ~> 0.29.0 (open-telemetry#1508)

Updates the requirements on [opentelemetry-exporter-otlp](https://github.com/open-telemetry/opentelemetry-ruby) to permit the latest version.

Updates `opentelemetry-exporter-otlp` to 0.29.0
- [Release notes](https://github.com/open-telemetry/opentelemetry-ruby/releases)
- [Changelog](https://github.com/open-telemetry/opentelemetry-ruby/blob/main/exporter/otlp/CHANGELOG.md)
- [Commits](open-telemetry/opentelemetry-ruby@opentelemetry-exporter-otlp/v0.28.1...opentelemetry-exporter-otlp/v0.29.0)

---
updated-dependencies:
- dependency-name: opentelemetry-exporter-otlp
  dependency-type: direct:production
  dependency-group: opentelemetry-deps-ruby
...

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

Author: dependabot[bot]

ruby/src/otel/layer/Gemfile

@@ -1,5 +1,5 @@
 source 'https://rubygems.org'
 
 gem 'opentelemetry-sdk', '~> 1.5.0'
-gem 'opentelemetry-exporter-otlp', '~> 0.28.1'
+gem 'opentelemetry-exporter-otlp', '~> 0.29.0'
 gem 'opentelemetry-instrumentation-aws_lambda', '~> 0.1.0'