Fix GPU b-field initialization

[shahor02]

The 12kA L3 field should be scaled wrt its own nominal rather than 30kA nominal Bz. Keep the old scaling for backward compatibility.

Don't merge until TPC CTF decoding with field Bz from the CTF itself is not ready, as this PR will crash low-field data decoding.

Example of difference:

// original field for -12kA:
auto f2 = o2::field::MagneticField::createFieldMap(-12000, -6000);
f2->GetBz(120, 120, -20)
(double) -2.0522978

// old gpu field initialized with -12kA:
fld.GetFieldBz(120, 120, -20)/0.000299792458f
(float) -2.00946f
// -> 2.1% difference

// field with this PR initialized with -12kA 
fld.GetFieldBz(120, 120, -20)/0.000299792458f
(float) -2.05180f
// -> 0.024% difference

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
async-2023-PbPb-apass5

[shahor02]

removed getNominalGPUBzOld since it will never be used.