chore!: remove ensure_installed='maintained'

Removes all support (and tests) for the parser category "maintained", as
this is no longer a useful category.

BREAKING CHANGE: replace `ensure_installed='maintained'` by an explicit
list of parsers, or use `'all'` (not recommended).

tests/unit/parsers_spec.lua

@@ -1,46 +0,0 @@
-local stub = require "luassert.stub"
-local parsers = require "nvim-treesitter.parsers"
-
-describe("maintained_parsers", function()
-  before_each(function()
-    stub(vim.fn, "executable")
-  end)
-
-  after_each(function()
-    vim.fn.executable:clear()
-  end)
-
-  it("does not return experimental parsers", function()
-    local old_list = parsers.list
-    parsers.list = {
-      c = {
-        install_info = {
-          url = "https://github.com/tree-sitter/tree-sitter-c",
-          files = { "src/parser.c" },
-        },
-        maintainers = { "@vigoux" },
-      },
-      d = {
-        install_info = {
-          url = "https://github.com/CyberShadow/tree-sitter-d",
-          files = { "src/parser.c", "src/scanner.cc" },
-          requires_generate_from_grammar = true,
-        },
-        maintainers = { "@nawordar" },
-        experimental = true,
-      },
-      haskell = {
-        install_info = {
-          url = "https://github.com/tree-sitter/tree-sitter-haskell",
-          files = { "src/parser.c", "src/scanner.cc" },
-        },
-      },
-    }
-
-    local expected = { "c" }
-
-    assert.same(parsers.maintained_parsers(), expected)
-
-    parsers.list = old_list
-  end)
-end)