chore: Fall back to legacy config loading in migration startup

[mhess-swl]

In the case where we don't have a genesis-network.json file present at startup, DiskStartupNetworks.genesisOrThrow(...) correctly falls back to loading config.txt etc. to produce the Network object. We need to do the same for migration cases, so this PR adds that same fallback to DiskStartupNetworks.migrationOrThrow().

Closes #17257

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (6f7e468)	96991	70490	72.68%
Head commit (3ac173d)	96992 (+1)	70493 (+3)	72.68% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#17433)	2	2	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.74%. Comparing base (6f7e468) to head (3ac173d).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #17433   +/-   ##
=========================================
  Coverage     68.74%   68.74%           
+ Complexity    22591    22590    -1     
=========================================
  Files          2610     2610           
  Lines         97208    97209    +1     
  Branches      10129    10129           
=========================================
+ Hits          66824    66828    +4     
+ Misses        26567    26565    -2     
+ Partials       3817     3816    -1     

Files with missing lines	Coverage Δ
.../com/hedera/node/app/info/DiskStartupNetworks.java	83.54% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

[tinker-michaelj]

LGMT, tyvm @mhess-swl !