`data_stream.namespace` description should use byte limit

[ebeahan]

Description of the issue:

The field description for data_stream.namespace lists a 100 character limitations. In the Elastic Agent codebase and Fleet docs, 100 bytes is the listed limitation.

The description stating a character limit is vague, since multibyte characters will reach the 100 byte limit before 100 characters. Recommend updating data_stream.namespace to specify a 100 byte limitation vs. 100 characters.

The Fleet docs call this multibyte nuance out by stating:

A namespace can be up to 100 bytes in length (multibyte characters will count toward this limit faster)