chore: update supported node versions to 14, 16, 18, and 20

[aabmass]

Expand test support for node versions up to 20. Drop support for other non-LTS versions (odd numbered). Node 21 is not passing tests right now https://github.com/google/pprof-nodejs/issues/283, so I will hold off on that for now.

[aabmass]

The system-tests are failing for Node 21. What's happening is the output off pprof is missing function name (it just says "anonymous"):

+ pprof -filefunctions -top -nodecount=2 time.pb.gz                                               
Main binary filename not available.
Type: wall                                                                                        
Time: Jan 31, 2024 at 10:07pm (UTC)
Duration: 10.03s, Total samples = 9274ms (92.48%)                                                 
Showing nodes accounting for 9126ms, 98.40% of 9274ms total                                       
Dropped 43 nodes (cum <= 46.37ms)
Showing top 2 nodes out of 5
      flat  flat%   sum%        cum   cum%                                                        
    8262ms 89.09% 89.09%     8269ms 89.16%  (anonymous) /tmp/tmp.5G3K6fnfvv/busybench/src/busybench.ts
     864ms  9.32% 98.40%      864ms  9.32%  (idle)

compare the the Node 20 version which has the function name busyLoop:

+ pprof -filefunctions -top -nodecount=2 time.pb.gz                                                
Main binary filename not available.
Type: wall                                       
Time: Jan 31, 2024 at 9:24pm (UTC)
Duration: 10.03s, Total samples = 9272ms (92.47%)                                                  
Showing nodes accounting for 8750ms, 94.37% of 9272ms total                                        
Dropped 44 nodes (cum <= 46.36ms)
Showing top 2 nodes out of 6
      flat  flat%   sum%        cum   cum%
    7910ms 85.31% 85.31%     7913ms 85.34%  busyLoop /tmp/tmp.RSf1fHb4u0/busybench/src/busybench.ts 
     840ms  9.06% 94.37%      840ms  9.06%  (idle)

I'm not sure why.. maybe I'll just save the fix until after the release is made, and only support up to Node 20

[aabmass]

~Maybe it is an issue with the source maps, since this is a .ts file~ After doing a lot of tests, I am stumped. Idk if this is a regression in V8 CpuProfiler we are using or if there is an issue in our mapping code on Node 21. It also seems possible that V8 has some new optimizations that are messing up the tests. I am going to create a new issue and say we only support up to Node 20 for now.

[aabmass]

Going ahead without node 21. PTAL

[tboevil]

It seems that one less file (tools/build/build.sh) has been modified and the prebuild binary will not be built. Is there any special reason for this? If not, I can submit an MR to add it.