Bump bundled theme versions for 5.6

[SergeyBiryukov]

Looking at the changed files since 5.5 (last theme release), each bundled theme has changes and should receive a new version.

Touching multiple themes:

• #47235, #49867, #50768, #51157, #51445, #51501, #51533, #51622.

Theme-specific tickets:

• Twenty Eleven: #51477.
• Twenty Thirteen: #50809, #51440.
• Twenty Sixteen: #51101.
• Twenty Seventeen: #51100.
• Twenty Nineteen: #49052, #51095, #51099.
• Twenty Twenty: #49797, #51098, #51148, #51335, #51574, #51648.

Previously:

• #50889 (5.5)
• #49743 (5.4)
• #48944 (5.3.1)
• #47172 (5.2)
• #46070 (5.1)
• #45792 (5.0.3)

[peterwilsoncc]

In 51919.2.diff​

• Combined 51919.diff​ (version numbers) and 51919.1.patch​ (underscores copyright years)
• Updated versions of CSS and JS files modified since last version bump in [48775] by running git diff --name-only --relative d8985baaadbd31cf84e98719295eb0cfe1b424de -- './src/wp-content/themes/**/*.css' './src/wp-content/themes/**/*.js' on a git checkout.
• Added stable tag to the twentytwentyone readme: 1.0.0
• A couple of legacy themes now use forked versions of genericons so I have used the reverse date format for those.
• No changes to the asset versions in 2021 as they use the theme version*.
• *except for a few locations hard coded to 1.0.0. These remain hard coded.

[sabernhardt]

I noticed one small typo in the Twenty Seventeen readme (​December 5)

[SergeyBiryukov]

In 49763:

Bundled Themes: Bump theme versions ahead of WordPress 5.6.

The following theme versions will be released in coordination with WordPress 5.6:

• Twenty Ten: 3.2
• Twenty Eleven: 3.6
• Twenty Twelve: 3.3
• Twenty Thirteen: 3.2
• Twenty Fourteen: 3.0
• Twenty Fifteen: 2.8
• Twenty Sixteen: 2.3
• Twenty Seventeen: 2.5
• Twenty Nineteen: 1.8
• Twenty Twenty: 1.6
• Twenty Twenty-One: 1.0

Props peterwilsoncc, hareesh-pillai, dkarfa, sabernhardt, poena, aristath, SergeyBiryukov.
Fixes #51919.

[SergeyBiryukov]

Reopening for backporting to the 5.6 branch after a second committer's review.

[helen]

In 49766:

Bundled Themes: Bump theme versions ahead of WordPress 5.6.

The following theme versions will be released in coordination with WordPress 5.6:

• Twenty Ten: 3.2
• Twenty Eleven: 3.6
• Twenty Twelve: 3.3
• Twenty Thirteen: 3.2
• Twenty Fourteen: 3.0
• Twenty Fifteen: 2.8
• Twenty Sixteen: 2.3
• Twenty Seventeen: 2.5
• Twenty Nineteen: 1.8
• Twenty Twenty: 1.6
• Twenty Twenty-One: 1.0

Props peterwilsoncc, hareesh-pillai, dkarfa, sabernhardt, poena, aristath, SergeyBiryukov.
Fixes #51919.
Merges [49763] to the 5.6 branch.