Fix SentryMonoBehavior instance being removed on scene switch in WebGL

[tustanivsky]

This PR adds DontDestroyOnLoad call to SentryMonoBehavior.Awake to ensure its instance won't be destroyed during the scene switch in WebGL builds.

Closes #1678

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Fix `SentryMonoBehavior` instance being removed on scene switch in WebGL ([#1754](https://github.com/getsentry/sentry-unity/pull/1754))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against f0f8266