Bump minimum target deployment to 15.0

[mojganii]

This PR bumps the minimum deployment target version to iOS 15.0.

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This change is 

[linear]

IOS-785 Drop support for iOS 14.2

[rablador]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mojganii)

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ios/MullvadVPN/Extensions/NSDiffableDataSourceSnapshot+Reconfigure.swift line 18 at r1 (raw file):

            reconfigureItems(itemIdentifiers)
        } else {
            reloadItems(itemIdentifiers)

I believe this reloadItems() function can be removed altogether.

[mojganii]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @rablador)

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ios/MullvadVPN/Extensions/NSDiffableDataSourceSnapshot+Reconfigure.swift line 18 at r1 (raw file):

Previously, rablador (Jon Petersson) wrote…

I believe this reloadItems() function can be removed altogether.

that's true.

[buggmagnet]

Reviewed 4 of 6 files at r1, 6 of 6 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @mojganii and @rablador)

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ios/MullvadTypes/Duration.swift line 14 at r2 (raw file):

/// drop-in replacement until the app supports iOS16. Ideally this whole file can
/// then be deleted without affecting the rest of the code base.
@available(iOS, introduced: 15.0, obsoleted: 16.0, message: "Replace with native Duration type.")

nit
Out of curiosity, did we need to bump this number (and al the others as well) when setting the minimum target to 15? 🤔

[rablador]

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mojganii)

[mojganii]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @buggmagnet)

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ios/MullvadTypes/Duration.swift line 14 at r2 (raw file):

Previously, buggmagnet wrote…

nit
Out of curiosity, did we need to bump this number (and al the others as well) when setting the minimum target to 15? 🤔

I did that to match with the version we're already supporting. it was not crucial to be changed.

[github-actions]

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