ci: remove RN 0.77.3 testing phases

[yedidyak]

Summary

• Removed the Android and iOS Buildkite testing phases for React Native 0.77.3.
• Deleted the now-unused RN 0.77.3 Buildkite job definitions.
• Left the RN 0.78, 0.84, and 0.85 testing phases unchanged.

Validation

• bash .buildkite/pipeline.sh > /tmp/rnn-buildkite-pipeline.yml plus rg "0\\.77\\.3|rn_77|android_rn_77|ios_rn_77" /tmp/rnn-buildkite-pipeline.yml — confirmed generated pipeline has no RN 0.77.3 phase references.
• ruby -e 'require "yaml"; pipeline = YAML.load_file("/tmp/rnn-buildkite-pipeline.yml"); abort "missing steps" unless pipeline.is_a?(Hash) && pipeline["steps"].is_a?(Array); puts "steps=#{pipeline["steps"].size}"' — generated Buildkite YAML parses, with 9 steps.
• git diff --check — no whitespace issues.

Not Run

• Full Buildkite/CI jobs were not run locally; this change only removes CI phase definitions.

Risks

• Low: scoped Buildkite configuration cleanup.
• Main risk is accidentally removing a still-needed RN 0.77.3 compatibility signal from CI.

Review Context

• This mirrors the existing pattern used by prior old-RN phase removals.
• No runtime source, dependency, or test code was changed.

Suggested Reviewers

• @markdevocht — authored the latest similar RN phase cleanup and recent RN pipeline updates for these Buildkite files.