Release langchain-ai21==0.1.6

Changes since langchain-ai21==0.1.5

ai21: fix core version (#22544)
ai21: fix ai21 unittests (#22526)
ai21: release 0.1.6 (#22508)
ai21: fix text-splitters version (#22366)
ai21[patch]: added license (#22153)
core, partners: add token usage attribute to AIMessage (#21944)
ai21: AI21 Jamba docs (#21978)
ai21[patch]: configuration fix (#21790)
core, standard tests, partner packages: add test for model params (#21677)
partners: Revert AI21 Labs docs scan feature (#21699)