Bazel warns about unsupported NDK

[jb-gcx]

In our CI, we see this warning when running djinni:

WARNING: The major revision of the Android NDK referenced by android_ndk_repository rule 'androidndk' is 26. The major revisions supported by Bazel are [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. Bazel will attempt to treat the NDK as if it was r21. This may cause compilation and linkage problems. Please download a supported NDK version.

So far we haven't run into hard issues, but addressing this would still be good. I'm guessing this is because of an older bazel version? NDK 21 is quite old and from what I can tell at least version 25b should be supported with current versions.