Update dependency tslint to v6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
tslint (source)	devDependencies	major	^5.8.0 -> ^6.0.0

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Release Notes

palantir/tslint

### [`v6.1.0`](https://togithub.com/palantir/tslint/blob/master/CHANGELOG.md#v610) [Compare Source](https://togithub.com/palantir/tslint/compare/6.0.0...6.1.0) - [enhancement] Upgrade to TypeScript 3.8, support new syntax ([#​4915](https://togithub.com/palantir/tslint/issues/4915)) - [docs][`no-object-literal-type-assertion`]() fix typo in rule description ([#​4911](https://togithub.com/palantir/tslint/issues/4911)) Thanks to our contributors! - Daniel Zwelling - Ryan Delaney ### [`v6.0.0`](https://togithub.com/palantir/tslint/blob/master/CHANGELOG.md#v600) [Compare Source](https://togithub.com/palantir/tslint/compare/5.20.1...6.0.0) This version has been marked as deprecated on NPM to encourage users to migrate to ESLint. - [bugfix] fix `enforce-trailing-newline` option in [`file-header`](https://palantir.github.io/tslint/rules/file-header/) rule ([#​4890](https://togithub.com/palantir/tslint/issues/4890)) - [bugfix] avoid writing to temp files with `writeFileSync` on macOS 10.15 ([#​4887](https://togithub.com/palantir/tslint/issues/4887)) - [bugfix] fix crash in [`completed-docs`](https://palantir.github.io/tslint/rules/completed/) rule when tag contents are undefined ([#​4895](https://togithub.com/palantir/tslint/issues/4895)) - [docs] fix example code of Radix rule ([#​4892](https://togithub.com/palantir/tslint/issues/4892)) - [docs] Add missing colon after `[error]` in testing rules documentation ([#​4903](https://togithub.com/palantir/tslint/issues/4903)) Thanks to our contributors! - Ivan Sorokin - Tanmoy Bhowmik - Meng Wang - Jay Bhagat - Aman Sharma - Josh Goldberg

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