Fix AbstractMCMC 5 compatibility

[devmotion]

Fix AbstractMCMC 5 compatibility introduced in #2115 (and thereby Turing 0.30.0) by switching all internal occurrences of init_params to initial_params.

Ref #2150 (comment)

[github-actions]

Pull Request Test Coverage Report for Build 7440855919

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 7279099457:	0.0%
Covered Lines:	0
Relevant Lines:	1396

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💛 - Coveralls

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (99a121c) 0.00% compared to head (b04db61) 0.00%.

Files	Patch %	Lines
src/mcmc/emcee.jl	0.00%	3 Missing ⚠️
src/mcmc/hmc.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2153   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          22      22           
  Lines        1396    1396           
======================================
  Misses       1396    1396           

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