(GSLC/GCOV) `noiseEquivalentBackscatter` datasets should be float32

Per the XML product specifications, the noiseEquivalentBackscatter datasets in GSLC and GCOV should be float32. Currently, they are float64.

Notes:

• RSLC uses dtype of float32 for noiseEquivalentBackscatter, per the specs. This issue is isolated to GSLC and GCOV.
• Reminder: When implementing this fix, please ensure that the statistics attributes (min/max/mean/std/etc) are updated to float32 as well. (See: #297 and #213 for reasons)

Specifications:

• GSLC spec: https://github.com/isce-framework/isce3/blob/db4f65ba231036c60b2809bb04dbcaf6021029bb/python/packages/nisar/products/XML/L2/nisar_L2_GSLC.xml#L1352-L1354
• GCOV spec: https://github.com/isce-framework/isce3/blob/db4f65ba231036c60b2809bb04dbcaf6021029bb/python/packages/nisar/products/XML/L2/nisar_L2_GCOV.xml#L1572-L1574

Example granule generated by ops with CRID 05016 - NISAR_L2_PR_GCOV_001_149_D_080_2005_QPDH_A_20251003T130952_20251003T131016_X05016_N_P_J_001:

cc: @hfattahi @gshiroma @seongsujeong

Start with the GSLC and GCOV XML specifications at the referenced lines, then compare their noiseEquivalentBackscatter handling with RSLC. Locate the GSLC/GCOV dataset-generation paths and verify that the dataset and its statistics attributes use float32; the fix is complete when both products match the specifications.