Replace reference to master

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

Author: daverodgman

BRANCHES.md

@@ -2,7 +2,7 @@
 
 At any point in time, we have a number of maintained branches, currently consisting of:
 
-- The [`master`](https://github.com/Mbed-TLS/mbedtls/tree/master) branch:
+- The [`main`](https://github.com/Mbed-TLS/mbedtls/tree/main) branch:
   this always contains the latest release, including all publicly available
   security fixes.
 - The [`development`](https://github.com/Mbed-TLS/mbedtls/tree/development) branch:
@@ -19,7 +19,7 @@ such as [`archive/mbedtls-2.7`](https://github.com/Mbed-TLS/mbedtls/tree/archive
 These branches will not receive any changes or updates.
 
 We use [Semantic Versioning](https://semver.org/). In particular, we maintain
-API compatibility in the `master` branch across minor version changes (e.g.
+API compatibility in the `main` branch across minor version changes (e.g.
 the API of 3.(x+1) is backward compatible with 3.x). We only break API
 compatibility on major version changes (e.g. from 3.x to 4.0). We also maintain
 ABI compatibility within LTS branches; see the next section for details.
@@ -103,7 +103,7 @@ CONTRIBUTING](CONTRIBUTING.md#backwards-compatibility).
 
 The following branches are currently maintained:
 
-- [master](https://github.com/Mbed-TLS/mbedtls/tree/master)
+- [main](https://github.com/Mbed-TLS/mbedtls/tree/main)
 - [`development`](https://github.com/Mbed-TLS/mbedtls/)
 - [`mbedtls-3.6`](https://github.com/Mbed-TLS/mbedtls/tree/mbedtls-3.6)
  maintained until March 2027, see

BUGS.md

@@ -7,7 +7,7 @@ Known issues in Mbed TLS are [tracked on GitHub](https://github.com/Mbed-TLS/mbe
 If you think you've found a bug in Mbed TLS, please follow these steps:
 
 1. Make sure you're using the latest version of a
-   [maintained branch](BRANCHES.md): `master`, `development`,
+   [maintained branch](BRANCHES.md): `main`, `development`,
    or a long-time support branch.
 2. Check [GitHub](https://github.com/Mbed-TLS/mbedtls/issues) to see if
    your issue has already been reported. If not, …

README.md

@@ -250,7 +250,7 @@ For machines with a Unix shell and OpenSSL (and optionally GnuTLS) installed, ad
 -   `tests/scripts/depends.py` test builds in configurations with a single curve, key exchange, hash, cipher, or pkalg on.
 -   `tests/scripts/all.sh` runs a combination of the above tests, plus some more, with various build options (such as ASan, full `mbedtls_config.h`, etc).
 
-Instead of manually installing the required versions of all tools required for testing, it is possible to use the Docker images from our CI systems, as explained in [our testing infrastructure repository](https://github.com/Mbed-TLS/mbedtls-test/blob/master/README.md#quick-start).
+Instead of manually installing the required versions of all tools required for testing, it is possible to use the Docker images from our CI systems, as explained in [our testing infrastructure repository](https://github.com/Mbed-TLS/mbedtls-test/blob/main/README.md#quick-start).
 
 Porting Mbed TLS
 ----------------