chore: add a ugnot constant

[deelawn]

Was starting to work on GNOT token locking and noticed we had no ugnot constant in our codebase. This PR adds a ugnot constant and replaces ugnot literals in most places.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.12%. Comparing base (56f8357) to head (e2d2cb4).
Report is 19 commits behind head on master.

Files	Patch %	Lines
gno.land/pkg/gnoclient/signer.go	0.00%	1 Missing ⚠️
gno.land/pkg/integration/testing_node.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2656   +/-   ##
=======================================
  Coverage   60.11%   60.12%           
=======================================
  Files         560      561    +1     
  Lines       74911    74919    +8     
=======================================
+ Hits        45036    45042    +6     
- Misses      26498    26501    +3     
+ Partials     3377     3376    -1     

Flag	Coverage Δ
contribs/gnodev	61.40% <100.00%> (ø)
contribs/gnofaucet	15.31% <ø> (+0.85%)	⬆️
gno.land	64.18% <85.71%> (+<0.01%)	⬆️
gnovm	64.13% <100.00%> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	62.03% <ø> (-0.03%)	⬇️

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[aeddi]

Hey, this is the newbie. I'm trying to help with the overflow of open PRs. :)
I just asked a question about one file, but other than that, it looks good to me. 👍