chore: remove regtest from release process in readme

[CharlieC3]

Description

Removes the Hiro PBC regtest network step from the release process in the README, as the regtest network has been decommissioned late 2021.

Type of Change

• API reference/documentation update

Does this introduce a breaking change?

No

[codecov]

Codecov Report

Merging #3004 (376c1dd) into master (4d4f67a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3004      +/-   ##
==========================================
- Coverage   83.08%   83.07%   -0.01%     
==========================================
  Files         235      235              
  Lines      188222   188224       +2     
==========================================
- Hits       156386   156369      -17     
- Misses      31836    31855      +19     

Impacted Files	Coverage Δ
src/vm/database/sqlite.rs	78.47% <0.00%> (-1.39%)	⬇️
testnet/stacks-node/src/run_loop/neon.rs	79.66% <0.00%> (-1.20%)	⬇️
src/net/dns.rs	91.11% <0.00%> (-0.58%)	⬇️
src/vm/analysis/errors.rs	69.84% <0.00%> (-0.51%)	⬇️
src/burnchains/bitcoin/blocks.rs	92.52% <0.00%> (-0.41%)	⬇️
src/burnchains/burnchain.rs	90.25% <0.00%> (-0.40%)	⬇️
src/chainstate/stacks/index/storage.rs	79.59% <0.00%> (-0.38%)	⬇️
src/vm/database/key_value_wrapper.rs	96.48% <0.00%> (-0.32%)	⬇️
testnet/stacks-node/src/event_dispatcher.rs	85.17% <0.00%> (-0.14%)	⬇️
src/chainstate/stacks/index/proofs.rs	78.72% <0.00%> (-0.09%)	⬇️
... and 21 more

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[blockstack-devops]

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