Bump minSdkVersion to 23 (Marshmallow).

RELNOTES=Our Android `minSdkVersion` is now 23 (Marshmallow). This follows the minimum of Google's foundational Android libraries, and we expect it to have no practical impact on users.
PiperOrigin-RevId: 791226165

Author: cpovirk

README.md

@@ -128,7 +128,7 @@ class by appending the class name to guava.dev. For example,
     on Linux, with some additional testing on newer JDKs and on Windows. Some
     features, especially in `com.google.common.io`, may not work correctly in
     non-Linux environments. For the Android flavor, our unit tests also run on
-    API level 21 (Lollipop).
+    API level 23 (Marshmallow).
 
 [guava-snapshot-api-docs]: https://guava.dev/releases/snapshot-jre/api/docs/
 [guava-snapshot-api-diffs]: https://guava.dev/releases/snapshot-jre/api/diffs/

android/guava/src/com/google/common/util/concurrent/Runnables.java

@@ -24,7 +24,7 @@
 @GwtCompatible
 public final class Runnables {
   /*
-   * If we inline this, it's not longer a singleton under Android (at least under the Lollipop
+   * If we inline this, it's not longer a singleton under Android (at least under the Marshmallow
    * version that we're testing under) or J2CL.
    *
    * That's not necessarily a real-world problem, but it does break our tests.

android/pom.xml

@@ -372,7 +372,7 @@
             <checkTestClasses>true</checkTestClasses>
             <signature>
               <groupId>com.toasttab.android</groupId>
-              <artifactId>gummy-bears-api-21</artifactId>
+              <artifactId>gummy-bears-api-23</artifactId>
               <version>0.12.0</version>
               <!-- TODO(cpovirk): In principle, it would make sense to *also* test compatibility with JDK 1.8, since guava-android also has JRE users. -->
             </signature>

guava/src/com/google/common/util/concurrent/Runnables.java

@@ -24,7 +24,7 @@
 @GwtCompatible
 public final class Runnables {
   /*
-   * If we inline this, it's not longer a singleton under Android (at least under the Lollipop
+   * If we inline this, it's not longer a singleton under Android (at least under the Marshmallow
    * version that we're testing under) or J2CL.
    *
    * That's not necessarily a real-world problem, but it does break our tests.