update readme for v4 and remove semver for version feature flags

[rochdev]

What does this PR do?

Update readme for v4 and remove semver for version feature flags.

Motivation

Minor improvements now that 4.0 was just released.

[github-actions]

Overall package size

Self size: 4.12 MB
Deduped: 58.25 MB
No deduping: 58.3 MB

Dependency sizes

name	version	self size	total size
@datadog/pprof	2.2.1	14.24 MB	15.12 MB
@datadog/native-iast-taint-tracking	1.4.1	14.85 MB	14.86 MB
@datadog/native-appsec	3.1.0	13.31 MB	13.32 MB
protobufjs	7.1.2	2.76 MB	6.55 MB
@datadog/native-iast-rewriter	2.0.1	2.09 MB	2.1 MB
@datadog/native-metrics	2.0.0	898.77 kB	1.3 MB
opentracing	0.14.7	194.81 kB	194.81 kB
semver	7.3.8	88.2 kB	118.6 kB
@datadog/sketches-js	2.1.0	109.9 kB	109.9 kB
lodash.sortby	4.7.0	75.76 kB	75.76 kB
lru-cache	7.14.0	74.95 kB	74.95 kB
ipaddr.js	2.0.1	59.52 kB	59.52 kB
ignore	5.2.0	48.87 kB	48.87 kB
import-in-the-middle	1.3.5	34.34 kB	38.81 kB
istanbul-lib-coverage	3.2.0	29.34 kB	29.34 kB
retry	0.10.1	27.44 kB	27.44 kB
lodash.uniq	4.5.0	25.01 kB	25.01 kB
limiter	1.1.5	23.17 kB	23.17 kB
lodash.kebabcase	4.1.1	17.75 kB	17.75 kB
lodash.pick	4.4.0	16.33 kB	16.33 kB
node-abort-controller	3.0.1	14.33 kB	14.33 kB
crypto-randomuuid	1.0.0	11.18 kB	11.18 kB
diagnostics_channel	1.1.0	7.07 kB	7.07 kB
path-to-regexp	0.1.7	6.78 kB	6.78 kB
koalas	1.0.2	6.47 kB	6.47 kB
methods	1.1.2	5.29 kB	5.29 kB
module-details-from-path	1.0.3	4.47 kB	4.47 kB

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[codecov]

Codecov Report

Merging #3148 (b0f4b9f) into master (0aa6c48) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3148      +/-   ##
==========================================
- Coverage   87.12%   86.95%   -0.17%     
==========================================
  Files         322      322              
  Lines       11764    11763       -1     
  Branches       33       33              
==========================================
- Hits        10249    10229      -20     
- Misses       1515     1534      +19     

Impacted Files	Coverage Δ
packages/datadog-instrumentations/src/jest.js	71.67% <100.00%> (-8.25%)	⬇️
packages/datadog-instrumentations/src/next.js	93.97% <100.00%> (ø)
packages/dd-trace/src/tracer.js	97.29% <100.00%> (ø)

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[pr-commenter]

Benchmarks

Comparing candidate commit b0f4b9f in PR branch readme-v4 with baseline commit 0aa6c48 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 446 metrics, 26 unstable metrics.