fix(testnet): add missing migration

[evilrobot-01]

Running try-runtime on the testnet runtime from main indicates a missing migration:

cargo b -r --features try-runtime
try-runtime --runtime ./target/release/wbuild/pop-runtime-testnet/pop_runtime_testnet.wasm on-runtime-upgrade live --uri wss://rpc2.paseo.popnetwork.xyz

2024-08-03T21:13:28Z ERROR runtime::frame-support] CollatorSelection: On chain storage version StorageVersion(1) doesn't match current storage version StorageVersion(2).
[2024-08-03T21:13:28Z ERROR runtime] panicked at /Users/frank/dev/pop-node/runtime/testnet/src/lib.rs:876:65:
    called `Result::unwrap()` on an `Err` value: Other("On chain and current storage version do not match. Missing runtime upgrade?")
thread 'main' panicked at cli/main.rs:324:10

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 3.30%. Comparing base (95bb4b5) to head (bd0e15a).

@@          Coverage Diff          @@
##            main    #155   +/-   ##
=====================================
  Coverage   3.30%   3.30%           
=====================================
  Files         30      30           
  Lines       6843    6843           
  Branches    6843    6843           
=====================================
  Hits         226     226           
  Misses      6617    6617           

Files	Coverage Δ
runtime/testnet/src/lib.rs	14.88% <ø> (ø)