Switch 'master' branch to 'main' and update strings where needed

[Kristina Hoeppner]

We are switching our 'master' branch to 'main' and thus also need to update some references to it. Out of scope is making changes to third-party plugins. At the moment, changes are also only made to branch references, but not to other references that require code changes, e.g. the web services 'master' switch.

[Mahara Bot (dev-mahara)]

Patch for "main" branch: https://reviews.mahara.org/12106

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/12106
Committed: https://git.mahara.org/mahara/mahara/commit/eb85cb1dd16c4fbc5e87471f3b5f1c25e23bd940
Submitter: Robert Lyon (<email address hidden>)
Branch: main

commit eb85cb1dd16c4fbc5e87471f3b5f1c25e23bd940
Author: Kristina Hoeppner <email address hidden>
Date: Thu Oct 14 14:49:05 2021 +1300

Bug 1947065: Rename referenced 'master' branch to 'main'

Change-Id: I269fc6d3fab91ac1491db216448995066aab5552

[Mahara Bot (dev-mahara)]

Patch for "21.10_DEV" branch: https://reviews.mahara.org/12148

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/12148
Committed: https://git.mahara.org/mahara/mahara/commit/4cfe9efdbb3e0644c1e451385e5b20bce21a44e6
Submitter: Robert Lyon (<email address hidden>)
Branch: 21.10_DEV

commit 4cfe9efdbb3e0644c1e451385e5b20bce21a44e6
Author: Kristina Hoeppner <email address hidden>
Date: Thu Oct 14 14:49:05 2021 +1300

Bug 1947065: Rename referenced 'master' branch to 'main'

Change-Id: I269fc6d3fab91ac1491db216448995066aab5552
(cherry picked from commit eb85cb1dd16c4fbc5e87471f3b5f1c25e23bd940)