Update dependency tslint to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
tslint (source)	devDependencies	major	5.20.1 -> 6.1.0

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Release Notes

palantir/tslint

v6.1.0

Compare Source

• [enhancement] Upgrade to TypeScript 3.8, support new syntax (#​4915)
• [docs]no-object-literal-type-assertion fix typo in rule description (#​4911)

Thanks to our contributors!

• Daniel Zwelling
• Ryan Delaney

v6.0.0

Compare Source

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 rule (#​4890)
• [bugfix] avoid writing to temp files with writeFileSync on macOS 10.15 (#​4887)
• [bugfix] fix crash in completed-docs rule when tag contents are undefined (#​4895)
• [docs] fix example code of Radix rule (#​4892)
• [docs] Add missing colon after [error] in testing rules documentation (#​4903)

Thanks to our contributors!

• Ivan Sorokin
• Tanmoy Bhowmik
• Meng Wang
• Jay Bhagat
• Aman Sharma
• Josh Goldberg

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