Avoid extending assertj Assertions in OpenTelemetryAssertions

Describe the bug
Because OpenTelemetryAssertions extends Assertions, it brings in assertThat(Object) (spelled assertThat(T)) from assertj. With a static import of assertThat, the extends prevents you from mixing OpenTelemetryAssertions with Truth. Since all the methods are static, it is unclear why it even extends Assertions. I didn't see a previous issue where this was discussed.

Removing the extends would be an API breakage, and it appears this class is stable. So a fix might have to be in the form of a new class, which is unfortunate.

Steps to reproduce

import static com.google.common.truth.Truth.assertThat;
import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;

...
// both method <T>assertThat(T) in OpenTelemetryAssertions and method assertThat(Object) in Truth match
assertThat(new Object()).isNotNull();

What did you expect to see?
Mix assertThat from multiple test libraries sources and have them only work on their specific types.

What did you see instead?
Compile error when using a static import.

Test.java:115: error: reference to assertThat is ambiguous
    assertThat(new Object()).isNotNull();
    ^
  both method <T>assertThat(T) in OpenTelemetryAssertions and method assertThat(Object) in Truth match
  where T is a type-variable:
    T extends Object declared in method <T>assertThat(T)
1 error

The workaround is to use a non-static import for OpenTelemetryAssertions and spell out OpenTelemetryAssertions.assertThat explicitly in tests instead.

What version and what artifacts are you using?
io.opentelemetry:opentelemetry-sdk-testing:1.36.0. Build environment is Gradle.

Environment
Compiler: AdoptOpenJDK 1.8.0_275-b01
OS: Debian unstable

Start with the OpenTelemetryAssertions class and reproduce the Java static-import example using Truth and OpenTelemetryAssertions.assertThat. Read the issue discussion about API compatibility and possible replacement classes; done means the reported ambiguous call is resolved without an unaddressed break for the stable existing API.