Define contents permissions on "dependabot validate" workflow

[mmorel-35]

Which problem is this PR solving?

• Related to [Feature]: Improvement of OpenSSF Scorecard Score #5815

Description of the changes

• Define contents permissions on "dependabot validate" workflow
• Was done with the help of https://app.stepsecurity.io/secureworkflow

How was this change tested?

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

[mmorel-35]

As it only check the json-schema for dependabot.yml, this level of permission shall be sufficient

@harshitasao, would you confirm ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.79%. Comparing base (49114db) to head (21e4d09).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5874   +/-   ##
=======================================
  Coverage   96.79%   96.79%           
=======================================
  Files         342      342           
  Lines       16525    16525           
=======================================
  Hits        15996    15996           
  Misses        341      341           
  Partials      188      188           

Flag	Coverage Δ
badger_v1	8.05% <ø> (ø)
badger_v2	1.81% <ø> (ø)
cassandra-3.x-v1	16.61% <ø> (ø)
cassandra-3.x-v2	1.74% <ø> (ø)
cassandra-4.x-v1	16.61% <ø> (ø)
cassandra-4.x-v2	1.74% <ø> (ø)
elasticsearch-6.x-v1	18.78% <ø> (ø)
elasticsearch-7.x-v1	18.84% <ø> (+0.01%)	⬆️
elasticsearch-8.x-v1	19.03% <ø> (ø)
elasticsearch-8.x-v2	1.80% <ø> (-0.02%)	⬇️
grpc_v1	9.52% <ø> (ø)
grpc_v2	7.14% <ø> (ø)
kafka-v1	9.74% <ø> (ø)
kafka-v2	1.81% <ø> (ø)
memory_v2	1.80% <ø> (-0.02%)	⬇️
opensearch-1.x-v1	18.88% <ø> (ø)
opensearch-2.x-v1	18.89% <ø> (ø)
opensearch-2.x-v2	1.80% <ø> (-0.02%)	⬇️
unittests	95.27% <ø> (ø)

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