fix: Update SystemExecutors to only use 4 threads, always (#1209)

This can cause contention issues on high-core-count machines

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- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-pubsublite/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
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Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the [samples format](
https://github.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

README.md

@@ -32,13 +32,13 @@ If you are using Maven, add this to your pom.xml file:
 If you are using Gradle without BOM, add this to your dependencies:
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-pubsublite:1.6.3'
+implementation 'com.google.cloud:google-cloud-pubsublite:1.7.0'
 ```
 
 If you are using SBT, add this to your dependencies:
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-pubsublite" % "1.6.3"
+libraryDependencies += "com.google.cloud" % "google-cloud-pubsublite" % "1.7.0"
 ```
 
 ## Authentication

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/SystemExecutors.java

@@ -33,9 +33,7 @@ private static ThreadFactory newDaemonThreadFactory(String prefix) {
 
   public static ScheduledExecutorService newDaemonExecutor(String prefix) {
     ScheduledThreadPoolExecutor executor =
-        new ScheduledThreadPoolExecutor(
-            Math.max(4, Runtime.getRuntime().availableProcessors()),
-            newDaemonThreadFactory(prefix));
+        new ScheduledThreadPoolExecutor(4, newDaemonThreadFactory(prefix));
     // Remove scheduled tasks from the executor as soon as they are cancelled.
     executor.setRemoveOnCancelPolicy(true);
     return executor;