fix(core): Avoid using Array.findIndex() as it is ES5 incompatible

[mydea]

Not sure if there is an easy way to polyfill this only for ES5 builds? But other than this, this should be straightforward enough.

Closes #7397

[lforst]

wondering if we should use something like browserlist....

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.24 KB (+0.12% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	62.95 KB (+0.11% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.87 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	55.87 KB (+0.12% 🔺)
@sentry/browser - Webpack (gzipped + minified)	20.62 KB (+0.12% 🔺)
@sentry/browser - Webpack (minified)	67.39 KB (+0.1% 🔺)
@sentry/react - Webpack (gzipped + minified)	20.65 KB (+0.12% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	48.54 KB (+0.18% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.51 KB (+0.23% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.76 KB (+0.21% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.17 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	37.19 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	61.18 KB (+0.09% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	54.35 KB (+0.04% 🔺)

[mydea]

It would be nice to have an easy way to just use this and have a polyfill be added only for the ES5 bundle. But let's merge this for now and maybe look into this later!