fix a bug in read_obs.F90 missing N21 for EARS and DBnet

[HaidaoLin-NOAA]

Description

This PR is to fix a bug in read_obs.F90 missing N21 for EARS and DBnet.

Fixes #506

Type of change

Please delete options that are not relevant.

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?
This fix has been tested in RRFS retrospective and RRFS-B real-time runs.

Checklist

• [ x] My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing tests pass with my changes
• Any dependent changes have been merged and published

DUE DATE for this PR is 11/8/2023. If this PR is not merged into develop by this date, the PR will be closed and returned to the developer.

[HaidaoLin-NOAA]

The regression tests on Hera are all good:
[Haidao.Lin@hfe08 build]$ ctest -j9
Test project /scratch1/BMC/wrfruc/hlin/GSI/GSI_2023/regression_2/GSI/build
Start 1: [=[global_3dvar]=]
Start 2: [=[global_4dvar]=]
Start 3: [=[global_4denvar]=]
Start 4: [=[hwrf_nmm_d2]=]
Start 5: [=[hwrf_nmm_d3]=]
Start 6: [=[rtma]=]
Start 7: [=[rrfs_3denvar_glbens]=]
Start 8: [=[netcdf_fv3_regional]=]
Start 9: [=[global_enkf]=]
1/9 Test #8: [=[netcdf_fv3_regional]=] ........ Passed 488.35 sec
2/9 Test #4: [=[hwrf_nmm_d2]=] ................ Passed 489.97 sec
3/9 Test #5: [=[hwrf_nmm_d3]=] ................ Passed 501.49 sec
4/9 Test #7: [=[rrfs_3denvar_glbens]=] ........ Passed 611.60 sec
5/9 Test #9: [=[global_enkf]=] ................ Passed 965.53 sec
6/9 Test #6: [=[rtma]=] ....................... Passed 1398.10 sec
7/9 Test #3: [=[global_4denvar]=] ............. Passed 1615.41 sec
8/9 Test #2: [=[global_4dvar]=] ............... Passed 1683.12 sec
9/9 Test #1: [=[global_3dvar]=] ............... Passed 1911.06 sec

100% tests passed, 0 tests failed out of 9

Total Test time (real) = 1911.09 sec

[hu5970]

The regression tests were finished on WCOSS2:

Test project /lfs/h2/emc/ptmp/Ming.Hu/gsi/GSI/build
    Start 1: global_3dvar
    Start 2: global_4dvar
    Start 3: global_4denvar
    Start 4: hwrf_nmm_d2
    Start 5: hwrf_nmm_d3
    Start 6: rtma
    Start 7: rrfs_3denvar_glbens
    Start 8: netcdf_fv3_regional
    Start 9: global_enkf

1/9 Test #8: netcdf_fv3_regional ..............***Failed  484.07 sec
2/9 Test #5: hwrf_nmm_d3 ......................   Passed  556.48 sec
3/9 Test #9: global_enkf ......................   Passed  612.84 sec
4/9 Test #4: hwrf_nmm_d2 ......................   Passed  667.25 sec
5/9 Test #7: rrfs_3denvar_glbens ..............   Passed  847.18 sec
6/9 Test #6: rtma .............................   Passed  1356.64 sec
7/9 Test #3: global_4denvar ...................   Passed  1766.99 sec
8/9 Test #1: global_3dvar .....................   Passed  1816.80 sec
9/9 Test #2: global_4dvar .....................   Passed  1819.77 sec

The netcdf_fv3_regional failed because of "Failure memthresh". It is not a fatal failure.

So, all tests passed.