ref: Replace string.substr with string.slice

[mydea]

substr is actually deprecated (see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr).

For our cases, slice() should work just as well - see this very informative answer: https://stackoverflow.com/a/31910656

I noticed this while playing around with typescript update, which complains about this being deprecated.

As far as I see, slice and substr would only perform differently when start !== 0, which we don't have anywhere.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.84 KB (-0.03% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.46 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.61 KB (-0.02% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	55 KB (-0.01% 🔽)
@sentry/browser - Webpack (gzipped + minified)	20.37 KB (-0.03% 🔽)
@sentry/browser - Webpack (minified)	66.54 KB (-0.01% 🔽)
@sentry/react - Webpack (gzipped + minified)	20.4 KB (-0.02% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.63 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.82 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.25 KB (-0.03% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.26 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.5 KB (0%)