Trim obsolete lines from Proguard configs.

Note that, even assuming that I'm right that our code does not need these lines, it's possible that _downstream users_ need these flags for _their own_ code. If so, they may see failures after this change, and they can solve them by adding `-dontwarn` lines like the ones that I'm removing.

Details on the lines I'm removing:

- The `Modifier` suppression in `annotations` was probably never necessary externally and likely became unnecessary internally after cl/510419614. That said, I can't seem to reproduce the warning they were suppressing even when I roll cl/510419614 back, so the actual "fix" may have been a change to Android or our build system.
- `addSuppressed` has been part of Android [since API Level 19](https://developer.android.com/reference/java/lang/Throwable#addSuppressed(java.lang.Throwable)).
- `@SafeVarargs` has been part of Android [since API Level 19](https://developer.android.com/reference/java/lang/SafeVarargs).
   - In both cases, I don't even think we needed to wait until our _`minSdkVersion`_ hit 19, just until we were confident that all our users were _compiling_ against 19+. That happened long ago.

RELNOTES=n/a
PiperOrigin-RevId: 743995869

Author: cpovirk

proguard/collect.pro

@@ -1,5 +1,3 @@
--dontwarn java.lang.SafeVarargs
-
 # The nested FieldSettersHolder class looks these up.
 #
 # We use -keepclassmembernames because we want for ImmutableMultimap and its

proguard/concurrent.pro

@@ -48,8 +48,6 @@
   static *** getCurrentEnvironment (...);
 }
 
--dontwarn java.lang.SafeVarargs
-
 # b/407533570
 -dontwarn com.google.common.util.concurrent.AbstractFutureState$VarHandleAtomicHelper
 # See post-submit commentary on cl/743198569 about b/408047495.