chore(nimbus): Split update firefox version by app and channel

[yashikakhurana]

Because

• Single monolithic PR for all Firefox version updates blocks independent merging like here and all tests run even when only one version changes.

This commit

• Splits Firefox version updates into 4 separate CircleCI jobs (Desktop Release/Beta, Fenix Beta, Fennec Release)
• Refactors external_integration_updater_script.sh to accept and arguments
• Each job creates individual PRs with descriptive titles
• Enables parallel execution and independent merging of version updates

Fixes #13993

[jaredlockhart]

Sick, thank you @yashikakhurana don't forget to update the 'required jobs' in the gh branch permissions 🙏

[yashikakhurana]

Sick, thank you @yashikakhurana don't forget to update the 'required jobs' in the gh branch permissions 🙏

sure sure, thanks @jaredlockhart