Travis-CI: Update default distro from Ubuntu Xenial to Bionic

Ubuntu Bionic 18.04 LTS will be around until 2023 while Xenial is only to
2021. Thus Bionic is much more suitable as the Travis-CI default build
environment for the 10.5 branch.

For now, still keep one Xenial build to ensure backwards compatibility.

Disable main.udf test until MDEV-21976 is addressed.

.travis.yml

 # vim ft=yaml
 # travis-ci.org definition
 
-dist: xenial
+dist: bionic
 
 git:
   depth: 2
@@ -116,6 +116,7 @@ jobs:
   # Define extra jobs that are run in addition to the main jobs matrix
   include:
     - os: linux
+      dist: xenial
       compiler: gcc
       env: DebPackages=xenial
       addons: &deb-addons
@@ -140,7 +141,6 @@ jobs:
         - env DEB_BUILD_OPTIONS="parallel=4" debian/autobake-deb.sh;
     - os: linux
       arch: s390x
-      dist: bionic
       compiler: gcc
       env: DebPackages=bionic
       addons: *deb-addons
@@ -153,33 +153,27 @@ jobs:
       # Some chosen ones with s390x and gcc
     - os: linux
       arch: s390x
-      dist: bionic
       compiler: gcc
       env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
     - os: linux
       arch: s390x
-      dist: bionic
       compiler: gcc
       env: CC_VERSION=7 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=binlog,binlog_encryption,encryption,rocksdb,versioning,rpl
       # Some chosen ones with s390x and clang
     - os: linux
       arch: s390x
-      dist: bionic
       compiler: clang
       env: CC_VERSION=7 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=archive,optimizer_unfixed_bugs,parts,sys_vars,unit,vcol,innodb,innodb_gis,innodb_zip,innodb_fts
     - os: linux
       arch: s390x
-      dist: bionic
       compiler: clang
       env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=csv,federated,funcs_1,funcs_2,gcol,handler,heap,json,maria,perfschema,plugins,multi_source,roles
     # Extra tests to ensure IBM Power and ARM 64 work
     - os: linux
-      dist: bionic
       arch: ppc64le
       compiler: gcc
       env: CC_VERSION=7 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
     - os: linux
-      dist: bionic
       arch: arm64
       compiler: gcc
       env: CC_VERSION=7 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main

mysql-test/unstable-tests

@@ -181,6 +181,7 @@ main.type_int                         : Modified in 10.4.12
 main.type_newdecimal                  : MDEV-20532 - Floating point differences
 main.type_ranges                      : MDEV-20532 - Floating point differences
 main.type_time                        : Modified in 10.4.12
+main.udf                              : MDEV-21976: Fails on Ubuntu Bionic 18.04, our Travis-CI platform
 main.union_crash-714                  : Modified in 10.4.12
 main.userstat                         : MDEV-12904 - SSL errors
 main.wait_timeout                     : MDEV-19023 - Lost connection to MySQL server during query