use a more sensible default value for epoch stability depth

[zeegomo]

use value for cardano mainnet as default https://cips.cardano.org/cips/cip9/

[coveralls]

Pull Request Test Coverage Report for Build 2003145443

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 9.281%

---

Files with Coverage Reduction	New Missed Lines	%
jormungandr-lib/src/interfaces/block0_configuration/epoch_stability_depth.rs	1	25.0%

Totals
Change from base Build 2000569170:	-0.001%
Covered Lines:	693
Relevant Lines:	7467

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2003153802

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 9.281%

---

Files with Coverage Reduction	New Missed Lines	%
jormungandr-lib/src/interfaces/block0_configuration/epoch_stability_depth.rs	1	25.0%

Totals
Change from base Build 2000569170:	-0.001%
Covered Lines:	693
Relevant Lines:	7467

---

💛 - Coveralls