Change `{TEXT2VEC,MULTI2VEC}_PALM` vectorizers to `x-google`

[tsmith023]

In the Vectorizers class:

• TEXT2VEC_PALM = "text2vec-palm" -> TEXT2VEC_GOOGLE = "text2vec-google"
• MULTI2VEC_PALM = "multi2vec-palm" -> MULTI2VEC_GOOGLE = "multi2vec-google"