Rename functions that generate random strings

[serpro69]

Should not have different names for functions that essentially do the same thing, so RandomService#nextString and RandomService#randomAlphanumeric should both be renamed to randomString

• nextString will need to be handled gradually, first by adding a deprecation warning and then removing in some future release
  • deprecation warning can also include a note about coming default length change RandomService#nextString has too big default value for length param #98
• randomAlphanumeric, which was added in Add randomAlphanumeric function to RandomService #92, can be simply renamed since it hasn't gotten out of a 1.9.0-SNAPSHOT yet