feat: replace "develop" non-branch name with "main"

[sitepark-schaeper]

the branch-name "develop" for non-feature-branches origins from an old git workflow that we no longer use. the appropriate value to change it to would be "main" as that is the git-branch that has replaced it.

"develop" variants of constants and methods have been kept but deprecated with the intention to remove them in a later major version.

this tries to keep compatibility as much as possible - the only exception is the return value of toString() methods.

[github-actions]

Unit Test Results

190 tests   190 ✔️  0s ⏱️
  12 suites      0 💤
  12 files        0 ❌

Results for commit fdc4f98.

[codecov]

Codecov Report

❌ Patch coverage is 70.96774% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.94%. Comparing base (36c6c03) to head (fdc4f98).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../com/sitepark/versioning/version/VersionTypes.java	58.82%	7 Missing ⚠️
src/main/java/com/sitepark/versioning/Branch.java	83.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #185      +/-   ##
============================================
- Coverage     63.03%   62.94%   -0.10%     
- Complexity      425      429       +4     
============================================
  Files            40       40              
  Lines          1550     1557       +7     
  Branches        250      253       +3     
============================================
+ Hits            977      980       +3     
- Misses          453      455       +2     
- Partials        120      122       +2     

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