Add [SequencedCollection](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/SequencedCollection.html) methods to ImmutableSortedSet.

[copybara-service]

Add SequencedCollection methods to ImmutableSortedSet.

There are a couple reasons that this is maybe kind of nice, and then there's my real motivation for doing it.

Reasons that this is maybe kind of nice:

• We add @DoNotCall and @Deprecated to the mutator methods, as usual.
• We refine the return type of reversed() to ImmutableSortedSet.

But again, those aren't my real motivation. (If they were my real motivation, you might notice, then there would be no reason for us to override getFirst() and getLast().)

My real motivation is that we are looking to give guava-android APIs that use Java 8+ types in their signatures, such as ImmutableSet.toImmutableSet. And as a result of some part of those changes, we start seeing errors in some of our Google-internal build rules. Those errors come because some the desugaring process is inserting its own versions of getFirst and friends, versions that refer to the desugar-provided copies of SortedSet and NavigableSet, which our build process then looks for but cannot find.

Since this CL is preparation for adding ImmutableSet.toImmutableSet, it constitutes further progress toward #6567

RELNOTES=collect: Added SequencedCollection methods to ImmutableSortedSet.