Bump uhlc to 0.8.0 - changing formatting of Timestamps as Strings

[JEnoch]

uhlc 0.8.0 brings this change: atolab/uhlc-rs#16 which changes the default conversion of NTP64 and Timestamp into a String, formatting the time as an unsigned integer in decimal format. This conversion is lossless and bijective.

In addition, the JSON structure returned by the REST plugin is changed:

• from:

  { 
    "key": "demo/example/zenoh-rs-put", 
    "value": "Put from Rust!", 
    "encoding": "text/plain", 
    "time": "2024-07-04T15:40:06.701984997Z/3936144fe7aaa47413ff22cfa8c88b73" 
  }

• to:

  { 
    "key": "demo/example/zenoh-rs-put", 
    "value": "Put from Rust!", 
    "encoding": "text/plain", 
    "timestamp": "7386690599959157260/3936144fe7aaa47413ff22cfa8c88b73" 
  }

[JEnoch]

I removed irrelevant unit tests in storages replication (in plugins/zenoh-plugin-storage-manager/src/replica/digest.rs) that were assuming RFC3339 format for Timestamps as Strings. Those tests where comparing Digests created from code with manually crafted Digests which were certainly copied from the created ones.