Remove >= from cabal-version (#1998)

Fixes warnings like:

```
Warning: test/testdata/testdata.cabal:3:27: Packages with
'cabal-version:
1.12' or later should specify a specific version of the Cabal spec of
  the form
  'cabal-version: x.y'. Use 'cabal-version: 2.0'.
```

Co-authored-by: Potato Hatsue <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: 3 people

Diff for: install/hls-install.cabal

@@ -6,7 +6,7 @@ author:        Many, TBD when we release
 maintainer:    [email protected]
 copyright:     2019
 build-type:    Simple
-cabal-version: >=2.0
+cabal-version: 2.0
 
 library
   hs-source-dirs:     src

Diff for: test/testdata/completion/completions.cabal

@@ -1,6 +1,6 @@
 name:          completions
 version:       0.1.0.0
-cabal-version: >= 2.0
+cabal-version: 2.0
 build-type:    Simple
 
 executable compl-exe

Diff for: test/testdata/definition/definitions.cabal

@@ -1,6 +1,6 @@
 name:          definitions
 version:       0.1.0.0
-cabal-version: >= 2.0
+cabal-version: 2.0
 build-type:    Simple
 
 library

Diff for: test/testdata/testdata.cabal

@@ -1,6 +1,6 @@
 name:                testdata
 version:             0.1.0.0
-cabal-version:       >=2.0
+cabal-version:       2.0
 build-type: Simple
 
 executable codeactionrename