feature: bump minimum required version to 2021.1

[bric3]

This implements the proposed changes in #116

• Removed the old compatibility workarounds.
• Fixed a few compilation warnings (mostly due to deprecation).

[lppedd]

I'll handle the remaining VCS stuff if you want.

https://github.com/lppedd/idea-conventional-commit/blob/6fa98d20b8b6d2af2c40140ef97d2132b1acff29/src/main/kotlin/com/github/lppedd/cc/vcs/InternalVcsService.kt#L124-L128

https://github.com/lppedd/idea-conventional-commit/blob/6fa98d20b8b6d2af2c40140ef97d2132b1acff29/src/main/kotlin/com/github/lppedd/cc/vcs/InternalVcsService.kt#L185-L191

[bric3]

Ah yes missed those, they were not listed in the problem view.

[lppedd]

Generally what I do is search for references to that annotation (I added it just for this reason).
But no worries, I'll do it and test it.

[lppedd]

I'll add commits in this PR.