deps: Downgrade opentelemetry to 1.51.0 (#4075)

v1.52.0 of
[opentelemetry-exporter-sender-okhttp](https://central.sonatype.com/artifact/io.opentelemetry/opentelemetry-exporter-sender-okhttp/1.52.0)
and
[opentelemetry-sdk-extension-jaeger-remote-sampler](https://central.sonatype.com/artifact/io.opentelemetry/opentelemetry-sdk-extension-jaeger-remote-sampler/1.52.0)
now requires OkHttp 5.x which is incompatible with customers still rely
on 4.x. Even though we don't use these two artifacts in client
libraries, they are included in the opentelemetry-bom. If a customer
uses v1.51.0 version of opentelemetry-bom and latest client libraries,
they would get enforcer errors.

Downgrade the opentelemetry version to 1.51.0 to give customers more
time to migrate their codebase to be compatible with Okhttp 5.x and
opentelemetry 1.52+.

Author: blakeli0

gapic-generator-java-pom-parent/pom.xml

@@ -33,7 +33,7 @@
     <gson.version>2.12.1</gson.version>
     <guava.version>33.5.0-jre</guava.version>
     <protobuf.version>3.25.8</protobuf.version>
-    <opentelemetry.version>1.52.0</opentelemetry.version>
+    <opentelemetry.version>1.51.0</opentelemetry.version>
     <errorprone.version>2.42.0</errorprone.version>
     <j2objc-annotations.version>3.1</j2objc-annotations.version>
     <threetenbp.version>1.7.0</threetenbp.version>