added v12.1 release notes (#2754)

docs/ReleaseNote_12.1.md

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+## V12.1.0
+This is a minor release that fixed some bugs introduced in the v12 release.
+
+## What's Changed
+* Fix typos by @luzpaz in https://github.com/flameshot-org/flameshot/pull/2705
+* rename Imgur API Key to Imgur Application Client ID by @thehunmonkgroup in https://github.com/flameshot-org/flameshot/pull/2719
+* fix issue about externalWidget launchapp by @Alaskra in https://github.com/flameshot-org/flameshot/pull/2698
+* Fix size not appearing on size tool when started from launcher by @borgmanJeremy in https://github.com/flameshot-org/flameshot/pull/2734
+* Added option to pin menu to close pin by @borgmanJeremy in https://github.com/flameshot-org/flameshot/pull/2736
+* disable option to launch on start by default by @borgmanJeremy in https://github.com/flameshot-org/flameshot/pull/2735
+* Fix magnify windows by @borgmanJeremy in https://github.com/flameshot-org/flameshot/pull/2746
+* DesktopFileParser only reads .desktop files @wd5gnr https://github.com/flameshot-org/flameshot/pull/2742
+
+## New Contributors
+* @cliffcoffee made their first contribution in https://github.com/flameshot-org/flameshot/pull/2726
+* @thehunmonkgroup made their first contribution in https://github.com/flameshot-org/flameshot/pull/2719
+* @wd5gnr made their first contribution in https://github.com/flameshot-org/flameshot/pull/2742
+
+**Full Changelog**: https://github.com/flameshot-org/flameshot/compare/v12.0.0...v12.1.0