Update readme txt file for all theme

[dkarfa]

Update year

[SergeyBiryukov]

Good catch! Since it's a non-code change, and all the default themes are changing anyway, we can probably get this in 5.3, as a follow-up to [46676] / #48517.

We should also add a unit test for this, in the same way we check the year in license.txt since [28064].

[SergeyBiryukov]

48566.diff​ includes the bump for Twenty Seventeen and a unit test.

[SergeyBiryukov]

Looking at the file paths, Twenty Seventeen has README.txt, while all the other themes have readme.txt.

Twenty Seventeen's file might need to be renamed for consistency and for the unit test to pass on Unix systems.

[dkarfa]

Hey @SergeyBiryukov
May I need to update also in src/wp-content/themes/twentynineteen/style-rtl.css for "Twenty Nineteen is based on Underscores ​https://underscores.me/, (C) 2012-2018 Automattic, Inc.
Underscores is distributed under the terms of the GNU GPL v2 or later."

[dkarfa]

Cool
Replying to SergeyBiryukov:

48566.diff​ includes the bump for Twenty Seventeen and a unit test.

[SergeyBiryukov]

In 46719:

Bundled Themes: Update copyright year in readme.txt.

Add a unit test to ensure the year stays up to date.

Props dkarfa, SergeyBiryukov.
Fixes #48566.

[SergeyBiryukov]

In 46720:

Bundled Themes: Update copyright year in readme.txt for Twenty Twelve and Twenty Twenty.

Follow-up to [46719].

See #48566.

[SergeyBiryukov]

In 46721:

Bundled Themes: Update copyright year in readme.txt.

Add a unit test to ensure the year stays up to date.

Props dkarfa, SergeyBiryukov.
Merges [46719] and [46720] to the 5.3 branch.
Fixes #48566.

[whyisjake]

In 47629:

Bundled Themes: Update copyright year in readme.txt.

Add a unit test to ensure the year stays up to date.

Extends [46721] to 2020 and the 5.3 branch.

Fixes #48566.

[whyisjake]

In 47630:

Bundled Themes: Update copyright year in readme.txt. - Revert [47629]

Reverts [47629] as the tests will be updated, rather then the themes.

Props peterwilsoncc, whyisjake.
Fixes #48566.