[chore] Update OTTL hash function recommendations (#34359)

**Description:**

* FNV is a non-cryptographic hash function, which makes it inadequate to
replace cryptographic hash functions like SHA-1.
* SHA-256 is still a recommended hash function by NIST. I've added a
link to the official list of recommended hash functions as a citation.

pkg/ottl/ottlfuncs/README.md

@@ -1185,7 +1185,7 @@ Examples:
 
 - `SHA1("name")`
 
-**Note:** According to the National Institute of Standards and Technology (NIST), SHA1 is no longer a recommended hash function. It should be avoided except when required for compatibility. New uses should prefer FNV whenever possible.
+**Note:** [According to the National Institute of Standards and Technology (NIST)](https://csrc.nist.gov/projects/hash-functions), SHA1 is no longer a recommended hash function. It should be avoided except when required for compatibility. New uses should prefer a SHA-2 family function (such as SHA-256 or SHA-512) whenever possible.
 
 ### SHA256
 
@@ -1206,8 +1206,6 @@ Examples:
 
 - `SHA256("name")`
 
-**Note:** According to the National Institute of Standards and Technology (NIST), SHA256 is no longer a recommended hash function. It should be avoided except when required for compatibility. New uses should prefer FNV whenever possible.
-
 ### SpanID
 
 `SpanID(bytes)`