Feature #6107 - Making cover sheet optional per release

[nbhasin2]

Added Top level feature flag per version - Enable / Disable the cover sheet. Eg. We made a change and want to show it as part of cover sheet update. We require setting flag per version. We maintain a list of versions in the app on when to show the coversheet

[mozillamobilebot]

SwiftLint found issues

Warnings

File	Line	Reason
UpdateViewModel.swift	35	Colons should be next to the identifier when specifying a type and next to the key in dictionary literals. (colon)
UpdateViewModel.swift	1	Header comments should be consistent with project patterns. (file_header)
BrowserViewController.swift	447	if, for, guard, switch, while, and catch statements shouldn't unnecessarily wrap their conditionals or arguments in parentheses. (control_statement)
BrowserViewController.swift	1141	if, for, guard, switch, while, and catch statements shouldn't unnecessarily wrap their conditionals or arguments in parentheses. (control_statement)
BrowserViewController.swift	487	Prefer empty collection over optional collection. (discouraged_optional_collection)
BrowserViewController.swift	878	Prefer empty collection over optional collection. (discouraged_optional_collection)
BrowserViewController.swift	1381	Prefer empty collection over optional collection. (discouraged_optional_collection)
BrowserViewController.swift	2421	Force casts should be avoided. (force_cast)
BrowserViewController.swift	2154	Don't add a space between the method name and the parentheses. (no_space_in_method_call)
BrowserViewController.swift	46	Variables should not have redundant type annotation (redundant_type_annotation)
BrowserViewController.swift	2153	Variables should not have redundant type annotation (redundant_type_annotation)

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