chore: display warning on deprecated <splash> tag usage

[erisu]

Motivation and Context

Users may still use old tag.

Description

Display a warning to migate to the new preference tag when <splash> tag is used.

This only checks for slash tags in platform[name="android as global tags could still be used in other platforms.

Testing

cordova build

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1451 (f68faa7) into master (606e9c4) will decrease coverage by 0.10%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #1451      +/-   ##
==========================================
- Coverage   72.52%   72.41%   -0.11%     
==========================================
  Files          21       21              
  Lines        1740     1744       +4     
==========================================
+ Hits         1262     1263       +1     
- Misses        478      481       +3     

Impacted Files	Coverage Δ
lib/prepare.js	47.10% <25.00%> (-0.23%)	⬇️

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[NiklasMerz]

This works.

Looks like I get these warnings twice but I think this is ok