Add min API level of 26 for the named capaturing in javadoc

- Named capaturing is only supported since API level 26 (Oreo)
- Although the constructs are available since ICU 55 (Marshmallow),
  the API in Matcher wasn't added until API level 26, and there was
  no CTS before Oreo. Thus, Android only officially supports it since O.

Bug: 71727710
Test: m docs
Change-Id: Ib04f3fcfb831be50f4e77b4ca8c2952349e1db77