Remove "This function was added in Prometheus 2.0" (#10719)

Since Prometheus documentation is versioned, do not write down that a
specific function was added in Prom 2.0, for consistency.

Signed-off-by: Julien Pivotto <roidelapluie@o11y.eu>

docs/querying/functions.md

@@ -411,8 +411,6 @@ expression is to be evaluated.
 `timestamp(v instant-vector)` returns the timestamp of each of the samples of
 the given vector as the number of seconds since January 1, 1970 UTC.
 
-*This function was added in Prometheus 2.0*
-
 ## `vector()`
 
 `vector(s scalar)` returns the scalar `s` as a vector with no labels.