docs: remove Gitter from issue templates

[ersachin3112]

This PR contains a:

• [ ] bugfix
• [ ] new feature
• [ ] code refactor
• [ ] test update
• [ ] typo fix
• [x] metadata update

Motivation / Use-Case

remove Gitter from issue templates

Breaking Changes

No

Additional Info

No

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.29 :warning:

Comparison is base (c058a06) 88.76% compared to head (ccdeb63) 88.48%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #657 +/- ## ========================================== - Coverage 88.76% 88.48% -0.29% ========================================== Files 3 3 Lines 356 356 Branches 115 115 ========================================== - Hits 316 315 -1 - Misses 37 38 +1 Partials 3 3 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/webpack-contrib/postcss-loader/pull/657/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib)

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