fix: fix validation errors log message that calls join on a string

[torresxb1]

Issue #, if available:
aws/aws-sam-cli#3493

Description of changes:

• remove .join call on validation_errors, which is already a string
• replace deprecated LOG.warn with LOG.warning (https://stackoverflow.com/questions/15539937/whats-the-difference-between-logging-warn-and-logging-warning-in-python)

Description of how you validated changes:
Ran transform with debugger and the message was printed properly

Checklist:

• Add/update tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #2245 (352d5e5) into develop (e7a1496) will increase coverage by 0.74%.
The diff coverage is 96.58%.

@@             Coverage Diff             @@
##           develop    #2245      +/-   ##
===========================================
+ Coverage    93.58%   94.33%   +0.74%     
===========================================
  Files           90       95       +5     
  Lines         6124     6598     +474     
  Branches      1260     1331      +71     
===========================================
+ Hits          5731     6224     +493     
+ Misses         183      179       -4     
+ Partials       210      195      -15     

Impacted Files	Coverage Δ
samtranslator/model/lambda_.py	93.10% <ø> (ø)
samtranslator/plugins/globals/globals.py	99.05% <ø> (ø)
samtranslator/region_configuration.py	77.77% <63.63%> (-22.23%)	⬇️
samtranslator/model/api/api_generator.py	93.24% <75.00%> (-1.13%)	⬇️
samtranslator/model/api/http_api_generator.py	91.21% <75.00%> (+0.02%)	⬆️
samtranslator/__init__.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/dialup.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/feature_toggle.py	100.00% <100.00%> (+12.16%)	⬆️
samtranslator/intrinsics/actions.py	98.75% <100.00%> (+0.03%)	⬆️
samtranslator/metrics/method_decorator.py	100.00% <100.00%> (ø)
... and 34 more

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