Fix 8gev detector name in GDML

[tvami]

I am updating ldmx-sw, here are the details.

What are the issues that this addresses?

Resolves #1323

Check List

• I successfully compiled ldmx-sw with my developments
• I ran my developments and the following shows that they are successful.

When running the tests, this should have no visible effect at all

[tvami]

Log failure is according to expectation:

<  [ Process ] 1 : RunHeader { run: 1, numTries: 76625, detectorName: ldmx-det-v14, description: One e- fired far upstream with Dark Brem turned on and biased up in target
  ---
>  [ Process ] 1 : RunHeader { run: 1, numTries: 76625, detectorName: ldmx-det-v14-8gev, description: One e- fired far upstream with Dark Brem turned on and biased up in target